Ditto to the other posters. Kids' snorkeling equipment would be available at Wal-mart, K-mart, or your local sporting goods store. However, I don't think you need to bring it with you. The dive shops in Ambergris will have equipment that will fit your children. The masks have adjustable straps, and the flippers come in children's sizes. We used Ambergris Divers, and rented the snorkel equipment from them - if I remember it correctly, it cost $6.00 US per day. Just my opinion - but if your child is too small to fit into the rental equipment, he/she is probably too young to snorkel offshore.
We took our kids (ages 10 & 7) snorkelling at Hol Chan and Shark Ray Alley. Even though they are both good swimmers, they wore a life jacket around their waist. The dive master also attached a rope to one of those round life preservers. He held onto one end of the rope, and instructed the kids to hold onto the other end with one hand.
The only other safety tip that I have is to make sure the number in your group is small, so you or the divemaster can keep a close eye on your kids. I saw some cruise ship passengers at Shark Ray Alley, and there were about 20 people to one guide - scary!
Our family had a blast snorkelling together. I'm sure yours will too!