The tips are pooled together for all the wait staff; so it really doesn't matter that they won't be eating in the dining room. Also, if you remove their tips, the room stewards will not get their fair share.
Have they cruised before???? I ask, just because I have 2 teen boys who always eat with us in the dining room--that's where the best food is! They can get pizza and other things any time. Not telling you what to do; but, I think they'll miss out on some great food if they don't join you.