Wow! I have never seen an inflatable car seat before(!)

The babies seat is exactly what people use here, but I am not sure about your "big boy"s seat.

Here is a link to the local Automobile Association, where the rules and recommendations are written.

http://www.tcs.ch/main/de/home/auto_...ndersitze.html
It has a list of "advised" car seats.
It also says that from 7 years old, all they need is a booster - at Coop Bau & Hobby they are about 30chf.

Maybe go to a local babyshop, where you can buy child car seats, and ask them, or at your local police station.

Not just a question of "getting caught" with the wrong seat, but also a question of safety for your child with some of the loons on the road here..