As a parent, I would argue that if I'm permitted to use a company car for personal use, for which I pay tax on, then I would fully expect to be allowed to put a car seat for my child in the car and to choose if this was placed front or back. I would also accept that it was my responsibility to ensure that the car seat I fitted was appropriate for the make/model of car in question as you are correct that not all car seats are suitable for every car.

As an employer, I don't think you need to make a big deal out of this. I would simply put a statement in your Company Car Policy stating that it is the parents responsibility to provide an appropriate child seat at their own expense and that the Company will accept no liability for such equipment held in Company cars. I would state that such devices must be easily removable to permit the vehicle to be readily accessible for business needs during which time all safety related features (e.g. front air bags) must be switched on.

Without being a car expert, I believe that with most modern cars, switching on/off the front air bags can be done from a button on the dashboard controls and therefore need not become a massive problem.

Thank you