An abrasion is a superficial rub or wearing off of the skin, usually caused by a scrape or a brush burn. Abrasions are usually minor injuries that you can treat at home.
Call a child doctor for abrasions which :

located close to the eye or on the face
embedded with debris such as dirt, stones or gravel
show signs of infection such as increased warmth, redness, swelling or drainage
cover a large area of the body (such as the chest or back or an entire limb)