Branched user permissions for editing content types
I think you may try Node Privacy by Role module.
First create three roles Parent Editors, Child Editors, Subchild Editors
Then while creating a node you will be asked about the permission to edit the node.
But this might not help you when you want to protect nodes per user.

There is also another module Taxonomy Access Control Lite which governs access to nodes based on the taxonomy terms applided to nodes.
In this you can create terms for Parent Editors, Child Editors and Subchild Editors

Apart from that there is also good discussion here:

http://drupal.org/node/24073

http://drupal.org/node/285369