Trigger to Update the Custom Last Modified Date Field in the Parent Object When the Records in the Child Object is Updated Using Apex Trigger in Salesforce | How to get the last modified date of a custom object in apex trigger

   TRIGGER LastModifiedDateTrigger ON childObjTrigger__c (BEFORE INSERT, after INSERT, BEFORE UPDATE, after UPDATE) {

      Map<Id,Datetime> idLastModifiedDateMap = NEW Map<Id,Datetime >();

      List<Datetime> childIdList = NEW List<Datetime>();

      Set<Id> setIdList = NEW Set<Id>();

    IF(TRIGGER.isBefore){

        system.debug('trigger before event');

    }ELSE IF(TRIGGER.isAfter){

        system.debug('trigger after event');

        IF(TRIGGER.isUpdate){

            FOR(childObjTrigger__c temp : TRIGGER.new) {

              childIdList.add(temp.LastModifiedDate);  

            }

            system.debug('childIdList ' + childIdList);

            List<parentObjTrigger__c> prntList= [SELECT Id, LastModifiedDate__c, (SELECT Id, LastModifiedDate FROM childObjTriggers__r WHERE LastModifiedDate=:childIdList) FROM parentObjTrigger__c];

            List<parentObjTrigger__c> lastDate = NEW List<parentObjTrigger__c>();

            FOR(parentObjTrigger__c p1:prntList){

                FOR(childObjTrigger__c c1:p1.childObjTriggers__r){

                    system.debug('c1 ' + c1.LastModifiedDate);

                    p1.LastModifiedDate__c=c1.LastModifiedDate;                  

                    lastDate.add(p1);                   

                }

            }

            UPDATE prntList;

            system.debug('lastDate1### ' + lastDate);

        }

    }

}