The law in a nutshell:
Children under 7 years must travel in a child seat, and from 7 to 12 either a child seat or a seat belt is required. The law doesn't state where in the car the child seat must be, but the
Touring Club recommends the backseat. If it is in the front seat, the front airbag should be deactivated (a side airbag is less dangerous).
The two standard methods of deactivation are either permanently by a garage, or manually by the driver, indicated by a control lamp on the dashboard.
Also according to the TCS, the Smart company was among the first to introduce a proprietary child seat system that automatically disables the airbag every time the child seat is mounted. Might be worth considering, depending on price and alternatives.