We just got back from my 7th cruise and my kids 2nd. Their first was on
DCL Magic, and now we were just on
RCCL's Explorer of the seas. My boys preferred RCCL by far. Here is a thread that I posted about that trip, I thought it might help
http://www.disboards.com/showthread.php?t=750920
Obviously most of the people who post on this board will prefer DCL, but I have to say, we all preferred RCCL better.
Not knowing anything about your kids, I would say, if they are boys, or very athletic, then RCCL would be my recommendation, if they are girls who like princesses etc. or if the are more into quieter group activities then DCL is best.
It seemed the "get out there" theme is prevelent on RCCL ships, there is so much active stuff to do, even in the kids clubs, whereas on DCL is was much more watch movies, see the characters etc. not near as much active stuff, so it really depends on what your kids are like.