Aggression towards children
This is usually fear induced. Children can appear very different from adults, and sometimes very frightening from a puppy's point of view. Good socialization with many ages and types of children is, again, the way to prevent problems.
Unfortunately, children often enjoy teasing dogs, and some can be unintentionally cruel. If your puppy has met and had many pleasant encounters with a wide variety of children, he is less likely to feel threatened by the occasional bad experience. A well-socialized dog will often feel competent to deal with a situation where he is tormented.