It is the responsibility of the supervising adult to ensure that any child under 5 years old is seated in a federally-approved child car seat. Failure to do so could result in a $60 fine and 3 points against your driver's license.
Children 3 years old and younger must use a separate car-seat or the vehicle's built-in child seat.
Children 4 through 5 years must sit in either a separate car seat, a built in child seat or a seat belt, depending on the child's height and weight.
Children 5 years old or younger must be secured in a federally approved child restraint system.
Though not required, the DHSMV recommends children 12 years old and under to ride in the back seat of the vehicle when possible. This is due to safety concerns for air bag deployment, which can be dangerous for young children.
Children under 18 years old must be in a seatbelt.
You can view the child passenger safety law in Florida here.