If you want an all-inclusive place that's great for kids go to Anthony's Key in Roatan Honduras. Your kids will love it and the diving is better than Akumal. You have to fly Continental or Taca. http://www.anthonyskey.com/
If your 12-year-old changes her mind about diving, and you decide to blow off your frequent flier miles and your budget, this is the way to go: http://www.kidsseacamp.com/
I think we all enjoyed Anthony's Key the best out of those three trips.