Puppies can go through fear stages. You need to expose her to children in a safe way, where she can feel comfortable and have it be a good experience. Maybe start with older kids that will allow her to sniff them, and they can give her a treat. Avoid young kids that move quickly, and pat roughly. Let her get comfortable with older kids first. And with younger kids maybe have them just toss her a treat. Do not punish her for growling. She's telling you in the only way she can that she is scared or uncomfortable. Remove her from the situation. Gradually expose her to kids while keeping the interaction safe and comfortable for her. That may mean that little kids do not pat her right now. That's ok. Much better than having her be fearful, and a little kid pat too hard, and she growls or snaps and you punish which affirms that little kids=pain/bad things.