I don't speak French, but a simple Google search for "hotel laws occupancy France" will return cite after cite after cite about this happening. And no, kids do not make it murkier, no matter what is made up. There is no law or rule requiring a hotel accommodate kids when a guest just shows up with them having not included them in the reservation. You can see on TP and other sites that many people have been left stranded believing that fiction about kids. Kids are people and are included in most non-American room counts, just like the Marriott Website indicates when asking for the number of kids and ages. The suggestion that one should just enter the wrong number of guests, show up with the kids and force the hotel to accommodate might work, but if it doesn't the guest can be stranded. Especially if you get one of the many hotel clerks in Paris who lives to stand firm on this.

Guests should reach out ahead of time, be accurate about the number of guests and ensure the reserved room can accommodate the correct number of people, not just assume there is some special exception for children. There isn't.