I'd take St. John out of the mix and spend a 2nd night at Leverick or Anegada. I know you say it's a must for your 7, 10, and 13 year old but I doubt they will really know the difference between St. John and all the other islands.

For the kids, I'd try to hit Leverick on a night they have the BBQ and Jumbie dancers if you're there in season.