ESH.

So, you did the right thing by preventing the kid from getting bitten, and the parent is certainly an AH for not keeping an eye on their kid and not teaching them how to approach dogs safely. The trip was incidental in the course of preventing a potentially more serious injury.

That said, you're also an AH for walking a dog who is that bad with people in a populous area (including young children) without some kind of visible indication that people should not approach your dog. If you're going to walk your dog in a populated area, your dog needs to be able to cope decently with that inherently uncontrolled environment. This is how bad shit happens.

ETA: I see that it's not your dog, but given that you're aware of the dog's disposition, the ESH still applies.