Quite honestly, you'd much rather have my 7 year old daughter in there playing around your infant than my 3 year old. Since my younger DD was 1 she has been all over the place. She climbs, jumps, and pretty much knows how to run. We have a playground at our mall, and she needs to run her energy off just as much as the infant does. I'm not saying that the toddler has more rights than an infant, just that they have pent up energy too. Personally, I think that if infants are allowed in a play area, it should be a play area for infants only, not even toddlers. Anyone who is walking, not crawling, poses a threat to an infant, because they might step on their fingers.
If you had my DD3, my niece3, and my nephew 1 and had to choose one to have around your child in a playground it would be tough to say. My niece weighs more than my duaghter, so if she fell on your infant it would hurt more. My niece is also mean. So, while my daughter is more likely to step on your infant because she is running, my niece is more likely to hit your child because they are on the slide she wants to be on. And, my nephew is 1 1/2 and weighs over 30 pounds. He weighs mores than my daughter. And, while he's not going to run, he doesn't walk very well, and loses his balance easily. You'd much rather have my DD step on your infant's fingers than have my nephew plop down on your child.
It does sound like the OP had some rough kids in the playground that a CM should have removed. I agree that it should have been taken care of at the time. There was supervisor there the whole time we were there. But, I don't think it's Disney's fault. It is the fault of parents who don't supervise their children.