5 is the perfect age to start a child in the BVI. I took my son the first time when he was 5 and just took my granddaughter. She arrived on her 5th Bday. Snorkled everywhere and was jumpin from the top of a 4800 cat into the water. The best prep you can do is get them comfortable swimming face down in the pool with a snorkle. Other then that they will be a self entertaining unit in the BVI on a boat.
G