trigger MapMontsOnSchedule on OpportunityLineItemSchedule (after insert, before update) {
    
    // UPDATED: Must handle multiple children for a single parent
    // Instead of a single OpportunityLineItem, hold a list of them
    Map<Id,List<OpportunityLineItemSchedule>> MapMonths = new Map<Id, List<OpportunityLineItemSchedule>>();
    
    // Have a temp list ready for looping
    List<OpportunityLineItemSchedule> tempOlisList;
    
    // Now populate the months map
    for(OpportunityLineItemSchedule sch : trigger.new) {
        // Check if the map already has an entry for the parent Id (key)
        if(MapMonths.containsKey(sch.OpportunityLineItemId)) {
            // If it does, then update the list with the new value (so it will not override the previous value)
            tempOlisList = MapMonths.get(sch.OpportunityLineItemId);
            tempOlisList.add(sch);
            MapMonths.put(sch.OpportunityLineItemId, tempOlisList);
        } else {
            // Otherwise, we will create a new entry in the map with a list value of just the current iteration's OLIS
            tempOlisList = new List<OpportunityLineItemSchedule);
            tempOlisList.add(sch);
            MapMonths.put(sch.OpportunityLineItemId, tempOlisList);
        }
    }

    List<OpportunityLineItem> OppLineItemList = new List<OpportunityLineItem>();
    for(OpportunityLineItem oli:[
        Select id, January__c,February__c, March__c, April__c, May__c, June__c, July__c, August__c, September__c, October__c, November__c,December__c 
        From OpportunityLineItem 
        Where Id IN :MapMonths.Keyset()
    ]) {
        // Then, the following is the updated if statement inside your for loop
        if(MapMonths.containsKey(oli.id)) {
            
            // UPDATE: Because we have a list of children now, we will need to loop through all of them to assign values to each month before moving on to the next Opportunity Line Item
            // Create a for loop to go through the list of children
            for(OpportunityItemListSchedule olis : MapMonths.get(oli.id)) {
                
                // Create a switch statement to check what value is the Month
                switch on olis.Month__c {
                    // If the opportunity line item schedule's Month field was January
                    when 'January' {
                        // Then assign the opportunity line item's January field to the opportunity line item schedule's quantity
                        oli.January__c = olis.Quantity;
                    }
                    when 'February' {
                        oli.February__c = olis.Quantity;
                    }
                    when 'March' {
                        oli.March__c = olis.Quantity;
                    }
                    when 'April' {
                        oli.April__c = olis.Quantity;
                    }
                    when 'May' {
                        oli.May__c = olis.Quantity;
                    }
                    when 'June' {
                        oli.June__c = olis.Quantity;
                    }
                    when 'July' {
                        oli.July__c = olis.Quantity;
                    }
                    when 'August' {
                        oli.August__c = olis.Quantity;
                    }
                    when 'September' {
                        oli.September__c = olis.Quantity;
                    }
                    when 'October' {
                        oli.October__c = olis.Quantity;
                    }
                    when 'November' {
                        oli.November__c = olis.Quantity;
                    }
                    when 'December' {
                        oli.December__c = olis.Quantity;
                    }
                    // If it did not find any matches
                    when else {
                        // EDIT: do not put anything here for now
                        // You can leave this empty or put something in here to handle cases where the Month field is NOT the name of the month
                        // e.g. Month field was for some reason populated with 'Apple'
                    }
                }
            }
            OppLineItemList.add(oli);
        }
    }
    update OppLineItemList;
}