This depends on the age, height, and weight of your child. The American Academy of Pediatrics recommends that all children under 40 lbs have a child safety seat during the flight. But it's not necessarily a requirement. Children 0-2 can sit in a parent's lap without purchasing their own seat. Or, if they have their own seat, they can use a car seat or CARES harness. The CARES harness is for children 1-4 years old. The height limit is 40 inches and the weight limit is 44 lbs.