Before hand I would communicate about any serious food issues, allergies, gluten free, vegan etc.
I do think staying in the one country will ease your citizenship concerns.With different arrival times I would suggest everyone meets up at the charter base or your 1st night accommodations.
3 young people will have little problem enjoying themselves without much input - well maybe a little advice and oversight!
If they are arriving in STT - their Immigration issues will be taken care of before they leave the airport terminal and shouldn't present any difficulty clearing in to BVI. Leaving from the USVI would not be a problem then, however if you do plan a visit the SVI you will still have to clear C&I there FYI.