Hi Michael - I second (and third) the suggestions of the family-friendly all-inclusive resorts of Mexico. I am a Canadian who has moved to Playa del Carmen in the Mayan Riviera after many years of traveling here (not a pro diver). There are a TON of things to do with kids here, in the water and out. For a full exhausting and educational kid day, take them to Xcaret eco park. It's touristy, but that also means there is a ton of things to do and see, and it's really well done and organized. They can snorkel, do dolphins, float the underground river, see animals, culture, a fabulous aquarium... I can't say enough about it if you want inundate your kids with all that is the Yucatan in one day. Stay for the night show if they are old enough to stay awake that long.
And the diving is fabulous... that's why I live here. Don't pass up the opportunity to do a cenote cavern tour, OWD is all you need and a bit of a sense of adventure. (plus reasonable buoyancy control).
Suggestions....
Iberostar (any of them EXCEPT the one in Playacar, the beach is gone)... stayed there once, looked like a good kids setup.
Bahia Principe - near Tulum, a bit of drive, one of the biggest all-inclusives down here. I have stayed there more than once, still have friends who do when they are in the mood for a resort and not my house. The dive shop, Bahia Divers, is decent. There's usually super cheap deals here, but if you want to fit in a Cozumel dive, it is going to be a LONG day to get to Playa, the ferry, dive, and then home.
Really, any of them that advertise a kids club and have a few stars are going to look after you. They are all scattered up and down a stretch of coast that is about an hour's drive long.
It's kind of a pick-your-price-point and book kind of place.
One last personal opinion... don't stay in Cancun, travel south towards Playa del Carmen, Puerto Aventuras, Akumal, Chemuyil, Tulum.
