Searching for children without parents on elastic search


I inserted some child documnets into elasticsearch with mapreduce and I verified their parents while inserting them but right now, they're some parents with no child document, even though the children exists there. It looks like the connection between them is broken or not verified.

I was looking for a query to see which children has no parent but it just shows me which children has parent. Would you please tell me how I can search for that?