Get Child Custom Post Content on Single.php?

If I understand your question correctly, you try to just get the children and ignore the parent.
Dont know where

post_child => 0

comes from but I’d use 

post_parent => $parent_id

instead. In that case you have to catch the parent ID before you do any new queries.
If you do something like this at the beginning of your template:

$this_page_id = $post->ID;

you can refer later, in your new Query, to that id and just query child pages by using 

post_parent => $this_page_id

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