When you try this, the forum returns an error about not having permission to delete this reply.
1) I swear this used to work
2) If this is on purpose (a deliberate change was made), then maybe the "Delete Children" checkbox should be removed? Or better yet, don't even offer the option to delete replies you're not allow to delete.
(There's nothing more frustrating than being given a button to press to perform an action, then when you press that button, you get an error saying "you can't do that!")