I hope the Mods will move this to the Cozumel forum, you'll get a better response there.
The Park Royal might be perfect for you. It is an AI with average rooms and average food. But beware, very aggressive timeshare people on the property. They have a kids pool area that is away from the adult pool. I am not sure about a Kids Club, but I think they have one. Another nice thing for small children is the beach at Park Royal. It is a secluded cove with virtually no waves at all. The hotel is across the street from the beach, but there is a very nice little walkway under the street to the small cove/beach. The beach is a public beach, but is very private. There is also a nice restaurant for lunch right on the beach, it is owned by the Park Royal, so it is part of your AI package. Last time I checked, children under 12 were free.
For diving: Aquaworld is just to the north of Park Royals' beach & next door to that is a small dive op I used called Scuba Shack. It has changed owners since I used them, but I was very happy with them when I used them.
