snmhanson I'm curious what you mean when you say "Never sailed the SVIs and very little in the USVI. However, it seems that the SVIs don't offer very much in the way of onshore activities". Specifically, what onshore activities are you looking for? Understanding the age of your kids, it would appear you are not looking for beach bars.
Are you requesting restaurants, shopping, hiking?
FarCry: Being from the area, you could probably identify a number of activities on St John that would appeal to kids and keep them happy for a week's charter. snmhanson could start the charter in Red Hook, where there are a number of restaurants (including Subway), a dive shop, pastries, coffee, etc. available in addition the the market. Mollie Malone's makes a good breakfast stop before departing for Leinster Bay for the first 2 nights on a mooring. There you'd have swimming/snorkeling from the beach for the youngest or off the dinghy near Waterlemon Cay. There are hiking trails where you can see abandoned ruins, get a panoramic view of the Drake Channel and Tortola, or do the tour of the Annaberg sugar mill--well worth the time and small entrance fee.
The 3rd day might be spend backtracking along the north shore and exploring the beach scene there. There could be a fancy meal at Caneel Bay to break out of camping-on-the-boat mode or exploring the camp ground at Maho Bay, although I am not aware of their policy for boaters (but I think it's friendly). Then there's Trunk Bay (but beaching/launching the dinghy can get exciting.) The moorings to the east of Lind Pt or even off Caneel Bay resort offer an opportunity to dinghy into Cruz Bay for shopping and restaurants, in addition to checking out the National Park visitor center.
One way or another, they might work their way around to Salt Pond Bay for more beach activities and snorkeling, especially off the boat. There are short hikes past the salt pond to Drunk Bay with the assorted mystical figures lying on the rocks or the longer hike up to Ram Head. You could easily spend 2 days here, but you are eating on the boat. If Salt Pond Bay's limited moorings are not available, right around the corner are the Lameshur Bays, which I've never seen filled. If anchoring isn't too scary, they might venture into Coral Bay and check out the laid back scene there for another restaurant and shopping experience. On the way back, one could make a detour for Christmas Bay or maybe another restaurant stop at Cruz Bay, using the aforementioned mooring field, for the last night of the charter. The next morning is an easy, 20 minute jog back to Red Hook.
I took my 8 and 12 yr old grandsons on a charter to the US & British Virgins a couple of years ago and they loved it. We spent 4 of our nights on St John. I suggest reading up on the history to make the area even that much more interesting. The slave rebellion on St. John is not kid stuff, but it is interesting and might help us northerners understand a little more of the background of the locals. The pirate legacy is always a fun thing for the kids, who at 8 yrs, anyway, can read Treasure Island before they go.