Every dog - like every human - has their own individual personality. Some people claim that all dogs of a certain breed have the same personality traits (like "all pitbulls are aggressive") but that is absolutely not true.
Your dog clearly loves his family (or pack). He doesn't have to love every unknown child as much as that. He must understand that he cannot be rude or attack people, but he's free to like some more than others.
Whom he actually likes or doesn't like is impossible for us to say. You certainly could get a feeling for his preferences by observing his interactions with people.
- Some dogs, especially anxious ones, dislike overly lively or exuberant children. If your dog is ancious, tell the children they need to be quiet and move slowly or remove your dog from the scene if he shows signs of stress.
- Some dogs dislike certain types of people due to bad past experiences. My own dog is afraid of every man he encountes. Watch how your dog interacts with different people to notice such a patten.
- Some children go straight to petting the head of a dog. This is actually rude behavior and most dogs dislike having their head pet without getting an introduction from the person first. Tell the children to hold their hand out to be sniffed at, then let them pet the cheeks or neck of your dog before going for the forehead.
- Body language plays a very important role in a dog's life. Children with a shy and insecure body language might provoke more of a dominant behavior like jumping up. Childfen running away might provoke a playfull chase that frightens them even more. You should read the children's body language and regulate your dog's responses to it.
- Another matter concerning body language: If your dog is very protective he might react badly to certain gestures from people, like hugging you or playing with your daugter. He might feel entitled to chase the other child away and "claim" you. Typical protective gestures are squeezing between you and the other person and jumping at people, nudging them away from you.