I also find the argument "playing outside" misplaced. Of course, children have to go outside as well, play football, ride bikes, climb trees, etc. Nevertheless, that doesn’t mean that a dark 6m2 hole with a bed as a kids’ room is okay. I can only speak from my own experience, but as a teenager we often hung out at friends’ places and did whatever. Most of the time we were wherever we were least disturbed or where there was the most space. Of course, you can make it deliberately small so that all the neighborhood kids are not hanging out in your own house, but in my opinion that is not the point. I had a schoolmate who had to share a 12m2 room with his (7 years younger) brother. Of course, he was always outside, inside he felt like the ceiling was closing in on him. 13m2 is already okay, but under 11 I find it borderline, though it doesn’t have to be 25 either. A 140cm bed should fit in, and if there’s also a seating area, all the better.