Your kids are definitely too young to spend the money on a day room - so you are right about that. We went and did the digs tour. It was a stifling hot July day so that was really getting to us. The tour was ok, but would be long and boring for young kids. The tour guide tells some fake Atlantis story thru out the tour. You would probably enjoy it more to pay to get in and then walk thru yourselves - pointing out fish to the kids, etc.. It is not too exciting, but at least you can say that you have been to Atlantis! We were drooling over the grounds there, we were cruise newbies when went and did not have a clue that you could get a day room and use the pools and slides, duh!! We would have loved that.
