noun
booster car seat; booster seat
A child car seat designed for older children who have outgrown a full harness seat; uses the vehicle's seat belt. Wasei-eigo from English 'junior seat'.
wasei-eigo (Japanese-coined English), from junior seat
子どもが大きくなったので、チャイルドシートからジュニアシートに買い替えた。
Since my child has grown, I replaced the child seat with a booster seat.
ジュニアシートは6歳くらいから使用できます。
A booster seat can be used from around age 6.