Merge custom object records with the child records..!!

You have to do little bit enhancement here.

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<apex:page controller="CustomMerge">
<apex:form>
    <apex:pageBlock>
        <apex:commandButton value="merge" action="{!mergeRecords}"/>
        <apex:pageBlockSection>
            <apex:inputField value="{!childrecord.Parent__c}" label="Merge Into"/>
        </apex:pageBlockSection>
    </apex:pageBlock>
</apex:form>    
</apex:page>
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public class CustomMerge{
    public MergeObj__c childRecord{get;set;}
    public List<Batch__c> btList = new List<Batch__c>();
	public List<Batch2__c> bt2List = new List<Batch2__c>();
    public CustomMerge(){        
        childRecord = [Select id,name,Parent__c,(select id,name,MergeObj__c from Batches__r),(select id,name,MergeObj__c from Batches2__r) from MergeObj__c where id =: apexpages.currentpage().getparameters().get('id')];
    }
    public void mergeRecords(){
        if(childRecord != Null){
            MergeObj__c parentRecord = [Select id,name from MergeObj__c WHERE id=: childRecord.Parent__c];
            for(Batch__c bt : childRecord.Batches__r){
                bt.MergeObj__c = parentRecord.id ;
                btList.add(bt);
            } 
			for(Batch2__c bt : childRecord.Batches2__r){
                bt.MergeObj__c = parentRecord.id ;
                bt2List.add(bt);
            }
            update btList ;
			update bt2List ;
            delete childRecord ; 
        }   
    }
}