A nosebleed is caused by the inside of the nose being too dry plus the normal rubbing and picking a child does when his nose is blocked or itchy. Being hit in the nose can make it bleed. Even blowing the nose too hard can cause a nosebleed. It is very common for a child to have a nosebleed.
Here's how to stop the bleeding:
Here are some ideas to keep from getting nosebleeds: