I can't help much with the kids' activities, but for a private (or semi-private) charter to Anguilla, I would look into GB Ferries LINK. Unless something has changed, they leave from near the Dutch-side airport. For a little more island flavor, you can always take the public ferry from Marigot. This will be less cost than the GB Ferries and you will still end up at the same place. Once on Anguilla, you can hire a taxi for a tour and perhaps ask about kids activities. There are many nice resorts and beaches that are worth seeing, but that might not interest the kids all that much. Sorry I can't be more help with the kid's activities and a package trip for that but in the past when I have gone to Anguilla with young local St. Maarten kids, they seemed to enjoy the beaches and having a hamburger from one of the beachfront restaurants.

Good luck.

The post above mentioned Billy Bones Charters. You might contact them and see if they can customize something for you.

BILLY BONES