Children's room and bedroom - What size is recommended?

So now I’m throwing a few numbers into the mix without any context... 15m2 children's room in a 140m2 house is large, but 12m2 in a 180m2 house something doesn’t add up.

But for the children, only the absolute value matters. Just because the house is only 100m2 doesn’t make a 9m2 room more bearable than in a 300m2 house? Unless there is an additional playroom

156 sqm total living area 12 sqm master bedroom + separate dressing room 2x 15 sqm children's rooms, identical layout, without sloping ceilings

It also depends a bit on habits. For example, we all spend a lot of time in the living room/kitchen. So they are planned very generously for us. The children's rooms in our home are 12 sqm and 11 sqm. The 12 sqm room is quite long in shape, which I find somewhat unfavorable. Something a bit more square would be nice. But it works like this too. The 11 sqm room is a bit more square and currently still a guest room with 2 beds, a wall unit, a shelf, and a wardrobe. My husband's oldest regularly crashes there. His comment: better smaller than too generous. That way he doesn’t have to walk so far when the console is turned on. The kid really crashes in 5 sqm. Girls might be a bit different there. I grew up in 10 sqm. I didn’t find it bad. The 12 sqm in my parents’ new house felt huge then. My conclusion: Small children are happy with 8 sqm. The main thing is space to play and cleverly used storage options (loft beds, shelves for books at bed height, etc.). The older they get, the more minimalist boys become and the higher the demands for girls. An extra couch in the children's room was never a must for me and a piano would then belong in the common room for me.

Evolith how big is your living dining area?

My daughter currently has an 8 sqm children's room in the rental apartment (2 years old) That is not just a bit too small but way too small for us.
Personal opinion

The entire living-dining area is about 70 sqm.

Our son currently plays on maybe 10 sqm. A large Playmobil dragon castle included. Because of his loft bed, he has space below for stuffed animals and a cozy corner. However, he only has a changing table and no wardrobe. He doesn't need one yet. With a wardrobe, space of course gets tighter. I built him a play table with Ikea stuff. It is very effective for storing toys.

Width 3.90 depth 2.95 is also enough for the bedroom

Not if a bed with a 2-meter mattress still has a comfortable, that is, padded frame/headboard. Then 65 cm is extremely tight. Always remember that the elf walk does not always work due to illness.