Custom button on Opportunity to clone opportunity with its related List (object)

Create a new Detail Page Button on the Opportunity object.

Set Behavior to Execute JavaScript and Content Source to OnClick JavaScript

Paste the Below snippet of code in the window

{!REQUIRESCRIPT("/soap/ajax/39.0/connection.js")} 
{!REQUIRESCRIPT("/soap/ajax/39.0/apex.js")} 
var retStr; 
retStr = sforce.apex.execute("CloneOpportunitywithRelatedObject","cloneOpportunity", 
{oppId:"{!Opportunity.Id}"}); location.replace('/' + retStr); 


Create a new Apex class and paste the below snippet of code

global class CloneOpportunitywithRelatedObject {

    webservice static Id cloneOpportunity(Id oppId) //  pass parameters
    {
        List < Related_Object__C > relatedList = new List < Related_Object__C > (); 

// query all the fields you need to clone 
        Opportunity opp = [SELECT ID, CloseDate, StageName, Type, Name FROM Opportunity 
                                      WHERE Id =: oppId];
        Opportunity oppCopy = opp.clone(false, true);
/ /     Append the new opportunity name with Clone Text 
        oppCopy.Name = opp.Name + 'Clone';
        insert oppCopy;
        //cloning Related object record
        for (Related_Object__C  p: [SELECT Id, Customer_Part_Number__c FROM Related_Object__C 
                                                   WHERE Opportunity__c =: opp.Id]) {
            Related_Object__C relatedCopy = p.clone(false, true);
            relatedCopy .Opportunity__c = oppCopy.Id;
            relatedList.add(relatedCopy);
        }
        insert relatedList;
        return oppCopy.Id;
    }
}
Note : Add the detail Page button on the Opportunity Layout 


Enjoy Coding !!