Why not make only childless parents deletable? If this is a hardcoded impossibility, I'd see about replacing software. Expandable/collapsible posting would be an improvement too. A better mousetrap is always possible, and if the one you need doesn't exist, build your own.

Context matters!

The "comment deleted" box offers context to the deleted comment's child comments. If we removed the "comments deleted" box, we would have child comments simply floating without a parent comment making it very difficult to follow the thread.