We went on RCL Sovereign of the Seas last year (4 day cruise) and took our four children. We got two rooms - adults in one and kids in the other, but our kids are older (range in age from 9-19). We book inside cabins on a lower level due to price. I've found we've spent very little time in the room and prefer to save our money for other things.
We bought soda cards for the kids. It was around $20 for the four days for unlimited soda. I found my kids really didn't drink as much as I thought they would because they had to go to the bar to get it so I don't think I would do that again. They mostly drank the free lemonade and ice tea. We bought alcohol beverage cards. Got 10 drinks for about $44. Not sure if they still sell them.
We did find the rooms on RCl to be small. This year we booked with Carnival. Also, my younger children weren't thrilled with the kids program. They ended up hanging out with us most of the time. We have friends that have cruised both Carnival and RCl and they've said that their kids love the Camp Carnival program and they think the food is better on Carnival, although we all thought the food on RCL was excellent.
We're doing the W.C. this year. Did Bahamas last year and really were not crazy about it. Loved Coco Cay but was not impressed with Nassau. From reading these boards it does look like shore excursions for smaller children are limited in Costa Maya and Belize. You might want to check into that some more before deciding on which to go to.
Hope this helps.