What does the "double" staircase on the upper floor mean?
What is the backpack in the bathroom next to the bathtub?
As I said, I would move the sauna to the upper floor, green the garage roof, and create an exit from the upper floor onto the greened garage roof – that way you can go outside after the sauna, but otherwise don’t need anything extra. You will urgently need the storage room downstairs for bikes, children's vehicles, and if a mini workshop is to be included as well, it will be tight anyway.
Then I would consider placing the bedroom in the NE and the bathroom in the NW (or vice versa), with both children above the dining/living area.
Am I seeing this right, is there another tiny air space from the hallway to the upper floor? What is that supposed to achieve besides noise transmission? I am a fan of open spaces, we have one ourselves, but if it’s no bigger than a shoebox, it just looks ridiculous and makes you feel like you’re standing in a chimney. With children, I would rather avoid that because noise transmission then weighs more heavily and this tiny air space otherwise brings no visual or living benefits.