Hi Lukasz,

I found the below code that could help writing code that clones records, can you try checking them out:

>> https://developer.salesforce.com/forums/?id=9060G000000UZsiQAG

Here is the code that is mentioned in one of the links for quick reference:
 

class AccountDeepCloneUtil 
{
    public static void cloneAccount(Account acc) 
    {
        Map<Opportunity,List<OpportunityLineItems>> OpportunityLineItemsMapping = new Map<Opportunity,List<OpportunityLineItems>>();
        List<Opportunity> newOpportunityList = new List<Opportunity>();
        List<Opportunity> oppList = [Select Id, Name, ... (Select Id, ... from OpportunityLineItems) from Opportunity where AccountId = :acc.Id];
        Account newAcc = acc.clone(false, true); //do a deep clone
        //insert the account record
        insert newAcc;
        for(Opportunity opp : oppList){
            Opportunity newOpp = opp.clone(false, true); //do a deep clone
            newOpp.Account = newAcc.Id;
            newOpportunityList.add(newOpp);
            OpportunityLineItemsMapping.put(newOpp,opp.OpportunityLineItems.deepClone(false,false,false));
        }
        //insert opportunity
        insert newOpportunityList;

        for(Opportunity opp :OpportunityLineItemsMapping.keySet()){
            for(OpportunityLineItemoppLineItem : OpportunityLineItemsMapping.get(opp)){
                OpportunityLineItem.OpportunityId = opp.Id;
            }
        }

        insert OpportunityLineItemsMapping.values();
    }
}

Here he is cloning the account and respective child records.

I hope this helps and in case if this comes handy can you please choose this as best answer so that it can be used by others in the future.

Regards,
Anutej