We went on our own to Cinnamon Bay. I highly recommend this. We had 3 kids - 3, 6 and 9 at the time. They all snorkeled and loved it. This beach is very uncrowded, so we were essentially the only ones in the water. There were a few other people on the beach. Because of this, the kids were not intimidated by a lot of people swimming around them. It is very easy to get around either by rental car or taxi on St. John - Cinnamon Bay is simple to get to and has a little convenience store to buy snacks/drinks, it has restrooms and places to rinse off, etc.
Now a question for you... Where did you take your kids snorkeling on Grand Cayman? We are going with ours in February and I've been looking for a good easy place to take them snorkeling.
Thanks!