Problem: I would like to prevent my site Users from creating more than one child post for a given parent post.
Solution: Create a Form that creates new child posts. Create a View of child posts, filtered by post author, where the author is the same as the current logged-in User, and also filtered by post relationship as related items of the post where the View is shown. In the "no results found" section of the View of child posts, insert the Form to create new child posts. Now the Form is only shown to those Users who have not yet created a child post for the current parent post.
Relevant Documentation:
See the FAQ here:
https://toolset.com/documentation/post-relationships/how-to-create-custom-searches-and-relationships-for-users/
