So everyone builds according to their own needs. The children live in the house for 18 years, true, but I live there for 50 years. But in the first years, they don’t use the children's room anyway. Certainly not to the extent that 20 sqm are needed. Ours is now 2 and still sleeps with us...
As teenagers, the need for space decreases again. Then, for gaming / surfing / whatever, people lounge on the bed / couch or are out with friends...
The space is effectively used maybe for 10 years. At least that is my experience. And when the kids move out, I have 2x 20 sqm empty, but a tiny bedroom.
Everyone can build what they want for themselves, but 15 sqm for a children's room is by no means small. Not even in a 200 sqm house.