They are usually caused by dryness of the nasal lining plus the normal rubbing and picking that all children do when the nose is blocked or itchy.
Vigorous nose blowing can also cause bleeding.
Children who have nasal allergies are more likely to have nosebleeds because they rub and blow their noses more. Lean forward and spit out any blood. Have your child sit up and lean forward so he does not have to swallow the blood. Have a basin available so he can spit out any blood that drains into his throat. Swallowed blood is irritating to the stomach.