Hello Dave: We used the Caribbean Yachting Charts (from Nautical Publications) when we cruised the BVI's with our two kids. Not sure if you have cruised there before, but the distances between destinations are quite short and it will be a good 'training' ground for basic navigation. We also found a couple quick reference laminated guides on Caribbean fish and fauna that were helpful after snorkeling the area. Not sure if they have them state-side, but they should be in the marine stores in the Tortola area. Our kids also enjoyed two books by Fritz Seyfarth, Tales of the Caribbean and Pirates of the Virgin Islands. Short books that were quite entertaining.
They'll have a great time and get far more value out of the trip there then they would have by another week of school. Good luck!
Thalia - The Grimmett's Sailing Adventure!