Atkinson is an English-language surname. The name is derived from a patronymic form of the Middle English Atkin.
These are much more conservative than state laws, in general (for instance, everyone should be in a booster seat from about age 4 to at least age 8) but remain less conservative than some of the european standards (for instance, the European Child Safety Alliance recommends that children remain in rear-facing carseats until 3 years of age).
It varies in different states. In California the child must be 6 years old or weigh more than 60 lbs to ride in the back seat without using a bolster or booster seat. For cars that have lap only seat belts (no shoulder straps) in the back seat, the child can stop using the car seat at 40lbs.