My dog is child phobic - help!

I have a Labrador-sized crossbreed, Mollie, who came from a small rescue kennel two years ago, where she was estimated to be about two years old. Mollie has always disliked children, off the lead she will go out of her way to avoid them, but if cornered she has been known to growl. We have no idea of her history, but I'm sure she has been tormented in the past. I usually walk her in places where we are highly unlikely to meet anybody, but at present I can't drive as the roads here are so bad. We have a large field nearby where lots of people walk their dogs and it's all I can use until the weather improves.
Unfortunately, it's also full of kids playing in the snow. Many of them seem to have no idea how to approach a dog, and today a youngster of six or seven ran up to her before I could intervene and tried to pat her. She was taken aback and did growl at him; luckily the mother told the child off for running up to a strange dog but I could meet a less sensible mum next time. Mollie has never actually bitten anyone, and I really do not want to subject her to a muzzle. She is a young dog and needs her exercise, and even if I keep her on the lead there's no guarantee that children won't come running up, they seem to have no manners these days. What can I do?
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