Mainer,
You'll love Sosua. I had never snorkeled and was amazed at how easily I could float in the salty water. We never saw kids doing it when we were there but I am sure they will love it. I was able to find a lot of beach glass, in all colors, and sand dollars, quite close to shore. You'll find places to stand up pretty far out. But watch out for the sea urchins!!! They are pretty to look at but hurt when you get stung! Also, my husband got blisters from his flippers, and I didn't have a pair that fit right, so after a while we went without. It takes longer to get around, but when you are surrounded by beautiful fish you're in no hurry, anyway.
By the way, have you ever gone up to Washington County to some of the beaches there? Roque Bluffs and Jasper Beach are two of the best! Very chilly, but eagles and seals abound!
Have fun in Sosua!
Amity aka the mainegirl