Each seat needs to be secure independently. So that if you were to take out any one seat, the others would still be properly installed.
You might need different seats, but cheaper than a new vehicle.
Car-seat . Org or something like that has lots of posters that can give better advice on exact seats.
In general, backless boosters can be smaller than high back boosters, radians are known for being slim seats.
There are some tiny infant seats that might work well if you can play musical seats and pass down the older kids' seat