Boxers are very high energy dogs. I have known gentle boxers and aggressive boxers. As with all breeds, the breeder and the pup's parents must be carefully scrutinized.
Boxers got their name because they like to rear up on their hind legs and "box" (play usually) with their front legs. This inbred trait can be a real problem for those with children. While some individual boxers will be great with children, I, personally, would look at other breeds - or even a mix - that has less energy. Boxers can be a lot to handle.