I've raised over 70 chickens. A scratch from a rooster talon hurts but it's not bad. You'll often bleed when attacked by an aggressive roo so kids need to be protected against the mean ones. The spurs on the back of their legs is also pretty dangerous but you can take those off using pliers without harming the bird. You can't really train them not to be aggressive, unlike dogs, chickens can be trained but it takes a lot of dedicated work that's not very suitable for outdoor or backyard chickens. Be the alpha, chickens have a pecking order so a gentle but firm boot kick can be enough to tell the roo to back off. If you continue to have problems with the roo that makes it dangerous for kids, people, or the flock, please consider removing the rooster from the others via separation.