Phase 3. Booster Seats. After age 4 and 40+ pounds, children can ride in a booster seat with the adult lap and shoulder belt until the adult safety belt will fit them properly (usually when the child is 4'9 tall, 10 12 years old).MUST have a lap/shoulder belt to use a booster seat.hase 3. Booster Seats. After age 4 and 40+ pounds, children can ride in a booster seat with the adult lap and shoulder belt until the adult safety belt will fit them properly (usually when the child is 4'9 tall, 10 12 years old).