CMP

<aura:component controller="AccRelatedConC"
                implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global" >
    
    <aura:attribute name="PageHeading" type="String" default="Account and Related Contacts" />
    <aura:attribute name="accList" type="List"/>
    
    <aura:handler name="init" value="{!this}" action="{!c.doinit}"/>
    
    <!--Component Start-->
    <lightning:accordion>
        <aura:iteration items="{!v.accList}" 
                        var="acc">
            
            <lightning:accordionSection name="{!acc.Name}" 
                                        label="{!acc.Name}">  
     <lightning:tabset selectedTabId="two">
        <lightning:tab label="Contacts" id="one">
            <aura:iteration items="{!acc.Contacts}" var="con" indexVar="index">
               <li>contact {!index + 1} Name : {!con.Name}</li>
            </aura:iteration>
        </lightning:tab>
        <lightning:tab label="Opportunities" id="two">
            <aura:iteration items="{!acc.Opportunities}" var="opp" indexVar="index">
               <li>Opportunities {!index + 1} Name : {!opp.Name}</li>
            </aura:iteration>
        </lightning:tab>
    </lightning:tabset>
               
                
            </lightning:accordionSection>
        </aura:iteration>
    </lightning:accordion>
</aura:component>



JS


({
    doinit : function(component, event, helper) {
        var action = component.get('c.fetchAcc');
        action.setCallback(this, function(response){
            var state = response.getState();
            if(state === "SUCCESS"){
                var allValues = response.getReturnValue();
                console.log("allValues--->>> " + JSON.stringify(allValues));
                component.set('v.accList', allValues);
            }
            else if(state === "ERROR") {
                var errors = response.getError();
                if(errors){
                    if(errors[0] && errors[0].message){
                        console.log("Error Message: " + errors[0].message);
                    }
                }
                else{
                    console.log("Unknown Error");
                }
            }
        });
        $A.enqueueAction(action);
    },
    

})


APEX


public class AccRelatedConC {
  @AuraEnabled
    public static List<Account> fetchAcc (){
        return [SELECT Id, Name,(select id,name from contacts),(select id,name from opportunities) FROM Account];
    }    
}