Many dogs develop a fear of children, especially if they have experienced a loud or excited child, who has played rough with the dog, or scared them by loud screams.
Small children can be difficult for dogs to cope with unless they've had lots of prior experience.
A dog can feel comfortable around children of one age, but unsure of younger or older children. Unfortunately, fearful behaviours do tend to generalise quite quickly.
If your dog has developed a fear of children, you will need to gently reintroduce children in a way that makes the dog feel comfortable.
Start by making sure the child is seated and quiet and when your dog approaches, the child can hold out rewards or give her a toy. Build up your dog's association between fun things and children.
Eventually your dog will become more relaxed, but ensure the children are not be allowed to run around or be noisy.
Keep training sessions very short and fun and make sure next the children aren't causing any distress if they're shrieking or laughing while playing. If your dog is left alone during the day within earshot of loud children, you will need to make other arrangements. You might want to use a sound-effects CD of children and baby noises to speed up your dog's desensitisation to the squealing that all excited children make.