Since the topic has now somewhat drifted anyway, I will also add my two cents:
As @Welmy already wrote, the size of children's rooms is a matter of opinion, and I also think you can't narrow it down to a specific necessary size. A well-designed 12 sqm room can definitely appear and feel much larger than a poorly designed 18 sqm room. And with 5 children and a limited budget, there simply comes a point when you've reached the limit. Nevertheless, I always find it amusing to compare today’s conditions with back then. I also had a children's room that wasn't even 10 sqm – and yes, it worked. Of course it worked because there simply was no other option. But more than a bed, a desk, and a wardrobe wouldn't fit in there. After that, I had a passageway that was not even a meter wide. Everything that was called a hobby was outsourced. I have always enjoyed making music. Larger musical instruments in the room – no chance. A seating area to quietly chat with friends – no chance. Sure, it worked somehow – but it was far from ideal.
What I find simply terrible with some builders is that sometimes the proportions just don’t match: huge bedrooms are planned for a room that most actually only use for sleeping, but the children have to live in cramped spaces. But from a certain age, children do live in their rooms. They want to invite friends, they may have hobbies that require some space, and they spend a large part of their time in their room (no one wants to tell me that 13, 15, or 17-year-old kids spend the majority of their time with their parents in the living room). And they should do so. Not everyone is always out and about. The argument that the children will eventually move out only applies to a limited extent for me, since they don’t live with you for just 1, 2, or 3 years but around 20 years (+/-).
What I like is a solution like in [Nordanney]: If you plan a play corridor or an extra communal room for the children or other retreat options, then the pure children’s rooms can certainly be a bit smaller.
Our children each have a 20 sqm room in total, which certainly wouldn’t necessarily be necessary and is actually a luxury. But rather than wasting the space (which we have on the upper floor due to the ground floor) on a large hallway, a large bedroom, or a dressing room that wouldn’t be used anyway, I prefer to give it to the children.