Hi
Kirsten is right and running a car hire business we often get asked this question. I have discussed it with the gendermerie too. The French rules work more on the height of the child rather than the weight. As a basic rule of thumb, we tell our clients that they should use the seat as they would use in the UK.
On a personal note, I have ferried around many French kids and always insist that it’s my car and my rules. I’m sure they think I’m a battleaxe but I rather be that then be responsible for any injures they incurred by not using one. I know several of my children’s friends who don’t wear a seat belt let alone use a car seat in their cars. That, and having too many people in the car so there are not enough seat belts. I always insist that my kids have a seat belt on their own, i.e. not with someone on their lap. I once had this situation with the local centre de loisirs so now make it very plain that the kids must have the right car seat/seat belt when travelling in one of their vehicles. Apart from anything it’s the law, but there are many who think it is not a very important law so not a problem.