I like Jeff's suggestions above.
Another option: Our kids for many years went to sailing camp at the Chesapeake Bay MAritime Museum
(CBMM). They use JY-15s, which is a good platform that can be sailed with mainsail alone or with mainsail and jib as the kids get more advanced.
They have a nice low key program over there, based on a half-day format for 4-5 days. We always made it a family vacation -- where we'd spend the week tied alongside there at the MAritime Museum (they have inexpensive slips for members - -well worth the membership fee). I would do boat projects (this is fun for me) and my wife relaxed while the kids sailed in the camp. Alternatively, you can anchor out in the inner lagoon and save some $$.
They definitely had a mixture of age groups, and they can pair kids up by age and ability.
Another option would be West River Sailing Club, in Galesville Md. We've heard they have a good program too.