Please be VERY careful!!
For a dog that age that has never been around kids, and has had a bad experience, I would be VERY VERY careful!
My Buddy snapped at a kid once but that wasn't until after the kid had fallen on him twice!
Neither of mine have been around kids, and instead of trying to get them used to kids, I just keep them away from kids. That way everyone is safe!
(The kids are safe from bites and my dogs are safe from possibly being put to sleep if they bite someone.)
Maybe you can find someplace where kids are out playing (running, screaming... ya know...Kid stuff! )
You and Hero can sit near by and watch them and let ONE kid at a time come over and sit down Slowly by Hero and pet her.
It's always the quick actions/reactions of kids that spook a dog to bite or a group of them that surround the dog to pet it.
I think another big reason for bites is because when a small kid has a dog and they can jump on it, pull it's tail... etc. and the dogs is fine with it, they automatically think that they can do those things to any dog!
So you have to watch the dog, the kids and hope that their parents are responsible enough to teach their kids not to approach every dog they see.
I wish you Good Luck!!