R.J.
A friend of mine told me that there is a place in Pembroke where she takes her granddaughter all the time. She says it's great because you let the kids play and there is a place for adults to sit and chat and have coffee or whatever.
There is also a play place in Weymouth called the children's museum on Library Memorial Parkway. It's not really a gym, but it's a place kids can go and do different kinds of things, i.e. they have a little house in there that's a store where kids can play with play food and a play register and stuff, they have musical instruments, lots of things. If you have a library card, you can get discounted passes so you can get in for I think $5, I don't remember exactly, but you can call and find out.
Hope this helps!