If the place you are staying is an "all-inclusive" they will have a huge buffet for every meal, with lots of food choices. If your kids like pools, you will be set staying right there, probably. The hotels typically have people working there who can setup any kind of excursion you want. There will be brochures and such, also. Best just to talk with them, tell them what you want, and let them set it up - then TIP THEM. We went snorkeling one day - out on a sailboat - very nice and fun, then on a bus to see Mayan ruins another day (not my favorite and HOT, but my then husband thought it was the coolest thing).
We've been to Isla Mujeres four times, the last time it was just me and my 9 year-old daughter. (She ate pancakes and cheese pizza there, mostly.) The first time we went to Cancun, we took the ferry there the first day and from then on, we stayed right on the island. We rented a golf cart and drove around the island exploring. It's a nice whole day event, with several stops along the way - turtle museum, art park with sculptures, yes the dolphin swimming thing - and the scenery is fantastic.
It depends what you like, and how adventurous a tourist you are. You can stay right in the hotel complex and relax and enjoy, or get more active and go on excursions, or see more of the "real" Mexico, if you choose.