Agree with Fitzhughlaw on renting, given the number you have. Island Surf and Sail (BVIWatertoys.com) also has them (in kids sizes). We also always order paddleboards from them which should be fun for the kids. Everything is delivered to the boat before you arrive. We now have older teenagers, but did our first charter with a 6 and 9 year old, so hear are a few thoughts:

Food/Provisionsing - We usually provision through Riteway and get other liquor through Tico or Caribbean Cellars as needed
Easy Kids Meals:
Kraft Mac n cheese
Grilling Burgers and dogs
Grilled Cheese, PB&J
Grill Chicken with Jerk Seasoning, black beans and yellow rice
Bowtie Pasta with a Red Sauce and add Italian sausage
Make them drink lots of water!

Fun Things with Kids:
Go with the Pirate theme - read up before and bring a couple of books
Go to Michael Beans Pirate Show at Leverick Bay - have the kids dress up
Obviously the usual easy snorkeling spots (Indians, Caves - maybe put a hidden treasure there for them to find, Monkey Point, Cistern Point (Cooper - but also small reef just off the beach to the right of the restaurant), Kitchen Point (Dogs)
Jump off Willy T (at lunch - don't take them later). Food is actually good too.
Watersports at BitterEnd (i.e. Hobie Cats)
Feeding of the Tarpon at Saba Rock at 5 PM.
Hiking and Crawling through the Baths - avoid Cruise Ship days and maybe better to cab from Leverick or Spanish Town vs swimming in. Swim in the pool at Top of the Baths after
Get a drink or lunch at Hog Heaven and take in spectacular view of North Sound
Anegada - rent a safari truck for the day and go to Loblolly and Cow Wreck, see the Flamingos. Consider a dinner out on the sand (lobster for adults) at Anegada Reef Hotel
Hike to the Bubbly Pool at Diamond Cay (by Little Jost). Eat at Foxy's Taboo
White Bay Jost - beach day with games at Soggy, Hendo's, etc. We like picking up picking up pizza to go at Coco Loco. Corsair's Pizza in Great Harbor is good as well, but not inexpensive.

Don't stress. You will have a great trip!