I see two choices,
Option 1.
Low tech is to show a report link field on a form of all children which are not linked to parents. The user would then go into edit mode and using Copy and paste in grid edit, select any children by selecting the correct parent.
Option 2.
Contact me and I will set you up with a way to land the user on a report of orphan children. The user would click click click and have full access to search and Dynamic filters to attach children, and then link back to the parent to see the parent with its children now attached. That usually takes 1-2 hours to set up.
