You really have several options.
Chankanaab Park (sorry I always slaughter the spelling) is a Marine Life Park that offers the best snorkeling "from the beach" because they have their own man made lagoon. The entrance fee is $16 (you can get some discount coupons from the web) for adults....not sure how much for kids.
Your wife can snorkel and I have read they have play ground equipment etc for kids. We have not been there, but lots of people enjoy it from their reviews. I could not find a website for them specifically ...but here is a review site. http://www.tripadvisor.com/Attractio...Peninsula.html
Beach snorkeling in Cozumel is not the best because of the iron ore "something or other" in the water. You really need to get OUT to the reefs. There is a company called Fury Catamaran that does snorkeling excursions. You can find them on the web as well. Here is a website that does a lot of excursions in Cozumel. I have never used them, since we always do our own thing. But it will give you an idea of what is available.
And then there are the beach resorts like Paradise Beach that offer more of the beach experience.
Paradise Beach is our favorite. There is no entrance fee. Lounge chairs, tables, umbrellas, clean restrooms and changing rooms, showers are all free. All they ask is that you eat/drink $10 worth per person during your day. They also offer a $9 wristband that is optional IF you want to play on their non-motorized toys. These include an iceberg, 2 water trampolines, kayaks, snorkeling equipment and floats. (life vests also for those who need it) Our 2 boys get the wristband, but hubby and I do not. (we sit in the shade) Since this is "beach" snorkeling, it is not as good as "reef" snorkeling. If your wife is an avid snorkeler....she will know the difference. The drinks are safe. They use purified water and the food is good. Here is their website
The bottom line is that Cozumel is a safe and tourist friendly port that does not require a ship excursion. Taxis are everywhere and generally it is $12-15 (one way ) for a taxi of 4 to get to Chank. Park or Paradise Beach . And lots of taxis to take you back to the ship.
Just be sure to keep your watch on SHIP time.
Have a great cruise and a wonderful trip to Cozumel.