Problem:
The user would like to prevent deletion of a post if it has related posts to it.

Solution:
There no built-in way to do this. But, I can imagine at least two ways.

- Simple way(if users interact with the system only from the frontend). Wrap the delete link of a post inside a conditional shortcode. The conditions should check if the post has any related posts in different relationships.
- Custom code way(should work for frontend and backend). Create a hook on the before_delete_post action, and check if the post has related posts. If it does, redirect to a different page to prevent Toolset from deleting the post. Check this StackOverflow thread https://wordpress.stackexchange.com/questions/92155/how-to-prevent-a-post-from-being-deleted
You can use Toolset relationships functions to check for the existence of related posts:
- https://toolset.com/documentation/customizing-sites-using-php/post-relationships-api/
- https://developer.wordpress.org/reference/hooks/before_delete_post/

We have published a snippet that actually deletes the related posts when a post is deleted, it may inspire you if you want to implement the custom code solution https://kb.onthegosystems.com/code-snippet/automatically-delete-child-posts-when-parent-is-deleted/