noun
booster seat; auxiliary seat; baby seat
A seat used to boost a child's height, often in cars or at tables. Can also refer to an extra or auxiliary seat in general.
子供が小さいので、車に補助椅子を付けています。
Since my child is small, I have a booster seat installed in the car.
レストランで補助椅子を借りました。
I borrowed a booster seat at the restaurant.
チャイルドシート is a child safety seat for cars, often with a harness, while 補助椅子 is specifically a booster seat that raises the child to use the adult seatbelt.
Compound of 補助 (ほじょ, 'assistance, auxiliary') and 椅子 (いす, 'chair').