We have about 16.5 sqm per children's room. I consider that a good compromise between too small and too large. The child will certainly also be fine with 12 or 13... BUT if you even have a thought about a possible resale, do not build children's rooms smaller than 15 sqm. Also advice from our architect. Houses with small children's rooms are simply less in demand...
Then I'll just make one out of two children's rooms - the third children's room I will then sell as an office.
But we have the advantage that our two older ones will move later in the house, we still have two rooms left in the attic, each about 30 sqm, which will later become their youth rooms. Then they are far away from our own bedroom and we have our peace.
At first, it was important to us that we all live together on one level.
Regarding the living room, the advantage is that on the ground floor we really only have one open space for living/cooking + utility room + guest bathroom.