First of all check your own feelings: do you feel afraid that she might run and attack? And, if you do, she might be picking your feelings. Fear creates adrenaline rush, and dogs are capable of detecting as a smell, in fact loads of our biochemical bodily odours "speak" to our dogs, and they know our moods not by watching our facial expressions only. You are worried, your dog smells it - there must have been a reason!
If your dog waight something like 40 kg - two valerian tablets for her and 3 for you before your walk - just about not to make you and your dog sleepy. Meet children in the park, somewhere outside ( don't use interiors) and try to play ball close to them. If it were children you know, you could ask their mother to pull on them some of your clothes, it would help your dog to identify them as something familiar and thus frienly. If you would persuade some boy to kick the ball for your dog - it would be perfect. She will lose her fear through playing. Sometimes this sort of fear comes as a result of high sensitivity of the dog's ear drums, children are noisy, you can put cotton buds in her ears in case of such destraction during your walks. But, it also cold be her old memories, if she was born not with a professional breeder but in the family where small children were allowed to play with puppies. In this case you cannot train her.