Hi there, welcome.
Note that RCCL will not be doing the 3-day and 4-day Mexico Baja cruise after October 13th. The last 3-day is the Oct 10 cruise and the last 4-day is the Oct 13 cruise. After that, RCCL is moving the Monarch of the Seas to Florida.
I have not been on Carnival, but did have a great time on RCCL for the 3-day. I'll be doing it again for the last 3-day this coming October. From what I saw, there was plenty to do for kids. But, remember, the 3-day is basically a huge weekend party.