I've done heaps of volunteer holidays - always with an environmental focus, though (UK, Iceland, Thailand). Make sure that the company you go with has been around for a while, as there are plenty of shady companies that are happy to take your money, but don't give you much of an experience ar meet your expectations. Expect volunteer holidays to be expensive - just because you're donating your time doesn't mean they don't need money, as well. I've rarely seen pure volunteer holidays that accept children. I know that companies like Intrepid (with whom I've done a few standard trips) have specific trips for families, and that they often have human/environmental or animal volunteer work scheduled as a small part of the holiday - perhaps that would be a good option until you know it works for your family?

Good on you for even thinking about it! Best of luck.