I have a component that allows users to update a picklist value on all child records from the parent record page. They are able to change the picklist values, but it won't save them to the child records.
component

<lightning:card iconName="custom:custom41" title="{! 'Item List for ' + v.RIM__c.Customer_Name__c}">     

<div class="slds-form slds-p-horizontal_medium" role="list">
<div class="slds-form__row"><div class="slds-form__item" role="listitem">
<div class="slds-form-element slds-form-element_horizontal slds-is-editing">    
      <lightning:select name="selectItem" label="Apply Decision to Selected:" value="{!v.selectedDecision}" onchange="{!c.setDcn}">
               <option text="None" value="{!v.value}"></option>
               <aura:iteration items="{!v.itemDecisionList}" var="stat">
                     <option text="{!stat}" value="{!stat}"></option>                    
               </aura:iteration>                    
       </lightning:select>
</div></div></div></div>  <!-- BEGIN TABLE JUNK -->

    <table class="slds-table">
        <thead>
            <tr class="slds-text-title_caps slds-line-height_reset">
                <th class="" style="width:10px">
                    <ui:inputCheckbox aura:id="box3" value="{!v.selectAll}" change="{!c.onCheck}"/></th>
                <th class="slds-p-around_xxx-small" scope="col"><div class="" title="Type">Type</div></th> 
                <th class="slds-p-around_xxx-small" scope="col"><div class="" title="Transaction Amount">Amount</div></th>               
                <th class="slds-p-around_xxx-small" scope="col"><div class="" title="Decision">Decision</div></th>
            </tr>
        </thead>
        <tbody>

<aura:iteration items="{!v.relatedItems}" var="itm">
<tr>
    <td><div class="" style="width:10px">
        <ui:inputCheckbox aura:id="boxPack" value="{!itm.itemSelected}"/></div>
    </td>

    <td data-label="Type">
    <div class="slds-form-element__control" title="Type">
        <p class="slds-text-body_regular">{!itm.wrapItem.Transaction_Type__c}</p>
    </div></td>

    <td data-label="Transaction Amount">
    <div class="slds-form-element__control" title="Transaction Amount">
           <p><lightning:formattedNumber value="{!itm.wrapItem.Transaction_Amount_Detail__c}" style="currency" currencyCode="USD"/></p>
    </div></td>             

    <td data-label="Decision">
        <ui:inputSelect  class="slds-select spear-select" aura:id="selectItem" value="{!itm.wrapItem.Decision__c}" updateOn="change" >    
            <aura:iteration items="{!v.itemDecisionList}" var="stat">
                <ui:inputSelectOption text="{!stat}" label="{!stat}" value="{!stat==itm.wrapItem.Decision__c}" />            
            </aura:iteration>    
        </ui:inputSelect>
<!-- aura:if isTrue="{!itm.wrapItem.Status__c == ''}"><img src="https://spectatorblogs.imgix.net/files/2016/03/ex.jpg"/>
    </aura:if-->&nbsp;
    </td>               
</tr></aura:iteration></tbody><br/> </table>    <!--BEGIN BUTTONS-->

            <div align="center">
                <lightning:button variant="neutral" label="Nvm" onclick="{!c.cancel}" />
                <lightning:button variant="brand" label="GO!" onclick="{!c.changeDcn}" /> 
            </div>

    </lightning:card>
</aura:component>

controller.js

({
    doInit : function(component, event, helper) {
        var action = component.get("c.getAllRelatedItems");  
        action.setParams({
            "visitRecordId" : component.get("v.recordId")});
        action.setCallback(this, function(a) {            
            component.set("v.relatedItems", a.getReturnValue());});
        $A.enqueueAction(action); 
        helper.getPrepopulatedValues(component, event, helper);},

    onCheck : function(component, event, helper) {
        var selAll = component.get("v.selectAll");
        var newlst =[];    
        var allItems = component.get("v.relatedItems");
        for(var i in allItems){
            var space = allItems[i];                    
            space.itemSelected = selAll;             
            newlst.push(space);}
        component.set("v.selectedItems",newlst);
        component.set("v.relatedItems",newlst);},

    setDcn : function(component, event, helper) {
        var selcDcn = component.get("v.selectedDecision");
        var newlst =[];    
        var allItems = component.get("v.relatedItems");
        for(var i in allItems){
            var space = allItems[i];                    
            if(space.itemSelected === true){
                space.wrapItem.Decision__c = selcDcn;}
            newlst.push(space);}
        component.set("v.relatedItems",newlst);}, 
//please help! ======================================================
    changeDcn : function(component, event, helper) {
        var allItems = component.get("v.relatedItems");
        for(var i in allItems){
            var space = allItems[i];    
            //alert("Decision: " + space.wrapItem.Decision__c);
        }

        var action = component.get("c.saveDcn");
        action.setParams({"dcns": allItems});
        action.setCallback(this, function(response){
            var state = response.getState();
            if (state === "SUCCESS") {
                var listy = component.get("v.itemDecisionList");
                listy.push(response.getReturnValue());
                component.set("v.itemDecisionList", listy);
            }
        });
        $A.enqueueAction(action);


    }, 
//===================================================================    
    cancel : function (component, event, helper) {
        var action = component.get("c.getAllRelatedItems");  
        action.setParams({
            "visitRecordId" : component.get("v.recordId")});
        action.setCallback(this, function(a) {            
            component.set("v.relatedItems", a.getReturnValue());});
        $A.enqueueAction(action); 
        helper.getPrepopulatedValues(component, event, helper);
    },
})

controller.apxc

public class MassUpdateController {
    @auraEnabled
    public static List<wrapItemRecord> getAllRelatedItems(Id visitRecordId){
        List<wrapItemRecord> wrapItemRecordList = new List<wrapItemRecord>();
        for(RIM_Item__c itm : [select Id, Name, RIM__r.Name, Decision__c, Transaction_Amount_Detail__c, 
                               Transaction_Type__c, RIM__r.Customer_Name__c 
                               from RIM_Item__c where RIM__c = :visitRecordId]){
            wrapItemRecordList.add(new wrapItemRecord(itm));
        }
        return wrapItemRecordList;
    }
    @auraEnabled
    public static List<String> getDcnValues(){
        List<String> options = new List<String>();
        Schema.DescribeFieldResult fieldResult = RIM_Item__c.Decision__c.getDescribe();
        List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();
        for (Schema.PicklistEntry f: ple) {
            options.add(f.getLabel());
        }
        return options;
    }
    //==================================================================
    @auraEnabled
    public static RIM_Item__c saveDcn(RIM_Item__c dcns){
        UPDATE dcns;
        RETURN dcns;
    }
    //==================================================================

    public class wrapItemRecord{
        @auraEnabled
        public boolean itemSelected{get; set;}

        @auraEnabled
        public RIM_Item__c wrapItem{get; set;}

        @auraEnabled
        public boolean isDecided{get; set;}

        public wrapItemRecord(RIM_Item__c itm){
            itemSelected = false;
            wrapItem = itm;
            if(itm.Decision__c == null){
                isDecided = false;
            }else{
                isDecided = true;
            }
        }
    }
}

helper

({
        getPrepopulatedValues : function(component, event, helper) {
            var action = component.get("c.getDcnValues");
            action.setCallback(this, function(a) {            
                component.set("v.itemDecisionList", a.getReturnValue());
            });
            $A.enqueueAction(action);
        },
    })

**controller.apxc**:
public class MassUpdateController {
    @auraEnabled
    public static List<wrapItemRecord> getAllRelatedItems(Id visitRecordId){
        List<wrapItemRecord> wrapItemRecordList = new List<wrapItemRecord>();
        for(RIM_Item__c itm : [select Id, Name, RIM__r.Name, Decision__c, Transaction_Amount_Detail__c, 
                               Transaction_Type__c, RIM__r.Customer_Name__c 
                               from RIM_Item__c where RIM__c = :visitRecordId]){
            wrapItemRecordList.add(new wrapItemRecord(itm));
        }
        return wrapItemRecordList;
    }
    @auraEnabled
    public static List<String> getDcnValues(){
        List<String> options = new List<String>();
        Schema.DescribeFieldResult fieldResult = RIM_Item__c.Decision__c.getDescribe();
        List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();
        for (Schema.PicklistEntry f: ple) {
            options.add(f.getLabel());
        }
        return options;
    }
    //==================================================================
    @auraEnabled
    public static RIM_Item__c saveDcn(RIM_Item__c dcns){
        UPDATE dcns;
        RETURN dcns;
    }
    //==================================================================

    public class wrapItemRecord{
        @auraEnabled
        public boolean itemSelected{get; set;}

        @auraEnabled
        public RIM_Item__c wrapItem{get; set;}

        @auraEnabled
        public boolean isDecided{get; set;}

        public wrapItemRecord(RIM_Item__c itm){
            itemSelected = false;
            wrapItem = itm;
            if(itm.Decision__c == null){
                isDecided = false;
            }else{
                isDecided = true;
            }
        }
    }
}