They are usually caused by rupture of a small blood vessel on the lining of the lower inside of the nose. This is caused by dryness of the nasal lining plus the normal rubbing and picking that most children do. It often occurs at night, as children sleep on their abdomen and often rub their nose against the pillow.
Vigorous nose blowing can also cause bleeding.
Nosebleeds are especially common in the Sacramento area because of the dry weather and the large amounts of pollens and resultant nasal allergies. With the first nosebleed, a blood clot forms on the nasal lining and stops the bleeding. This may feel itchy. Rubbing or picking the nose can dislodge the clot and the nose then bleeds again. Have the child sit up and lean forward so that he does not have to swallow the blood.