C.O.
It sounds like you need to find new playmates. I have a 9 month old little boy as well, and while he is really social with his cousins (2 2years olds and 1 4 year old) he is not really playing with them as much as watching. I agree it is good for him to be around other kids and babies, but if you aren't enjoying yourself, find new friends. And just back slowly off the old ones. an "i am just too busy to make it" works well enough in my mind.
On a side note, I can understand your frustrations at the 6 year old. However, a 6 year old is so light years ahead of the other 2 babies, these play dates can't be fun for her after the initial baby-fascination wears off. It is not a surprise that she is acting out if she is basically left alone to play with 2 babies who she can't really be social with. It all sounds a little mis matched to me. Your frustrations are totally founded.
However all that being said, if you want to continue your friendship with the moms, you have to find a way to put up with the kids. You would expect the same for your son.