There is no law that requires an adult to sit next to a child that travels in a booster seat. A booster seat is for older children who no longer fit into a forward facing car seat. Depending on height and weight, children between the ages of 8 and 12 years old may stop the use of a booster seat and use a shoulder and lap seatbelt.
Booster seats work by allowing a regular seatbelt to fit a child properly. Each booster seat manufacturer will have its own weight and height limit for a booster seat. It is recommended by the National Highway and Traffic Safety Administration that children sit in the backseat until the age of 12 because it is the safest place.