@Randave Winston 

what's the point in showing and then running validation?

Why not simply hide those using advanced ref qualifier?

If you want to show and don't want them to select then do this

1) write onChange client script on that field and using GlideAjax verify if this is a parent or child

2) based on that show alert and clear the field

If my response helped please mark it correct and close the thread so that it benefits future readers.