If you are beach people, Cozumel offers several beach resorts that you can book on your own (without having to pay the extra price of going through the ship). Cozumel is very safe and tourist friendly.
We always go to Paradise Beach, because the boys like the non-motorized water toys. There is no "entrance fee" or "lounge chair" fee...they just expect you to eat and/or drink while you are there. There is an optional wristband that is $9 for the day if you (or your oldest) wants to play on the iceberg, water trampoline, kayak, snorkel or floats.
Understand that snorkeling anywhere "off the beach" in Cozumel is going to be limited at best.
Chank> Park is also another option. Adults are $16.00 a person (I don't know the child's rate) but they have a website on the net that can provide you with all the info. This is where people swim with the dolphins. For that particular experience, I would recommend you book it through the ship to make sure you get a spot. They give cruise ship passengers preference (from what I understand). CP has a nice play area for the kids and some good snorkeling. I don't know if that is extra for the snorkeling...but the website should have all that info.
Both are about the same distance from the ship which will cost you $12-14 one way (for a taxi of 4).