There is a seat belt test that all children under 13 years old can try. Sit them right against the backseat and see if their knees naturally bend at the edge without slouching. If their knees do bend naturally over the seat's edge, move to the next step. Put on their seat belt. The lap belt should rest on the upper legs. If it does, check that the shoulder belt is sitting across their chest without digging into their neck. If the belt rests on their shoulder, see if your child slouches or does not sit correctly and upright with the seat belt on. If your child fails any part of this seat belt fit test, have them ride in a booster seat until they can meet all of the steps.