12x9.6m 2 full floors, basement, attic, 4 children's rooms

and also the upstairs

Swap bedroom and kitchen. Are the two small rooms labeled [Büro] really necessary? On the upper floor, a second sink is missing in the bathroom.

[I started from the largest possible building envelope, as there is also the corresponding space requirement.] This approach may be logical but practically leads to tightly packed rectangles. Therefore, my suggestion for a counter-thesis. Just try an optimal floor plan arrangement without regard to the plot. Then you have a representation for comparison, showing how many (and which) corners it would pinch. At first, just as a site plan, without wall thicknesses and certainly without window sizes or installation channels. Simply so that there is a target-actual comparison. And then see what it inspires.

Swap bedroom and kitchen.
Do we really need the two little rooms labeled as office?
Upstairs there is a second sink missing in the bathroom

A good idea, if it makes sense on the property and, for example, a nice view of the garden is not lost. I also prefer living, dining, and kitchen areas arranged around a corner, because you don't disturb each other as much. With 4 children, someone will definitely want to watch TV or sit quietly on the sofa while noise is made in the kitchen. It's nicer around the corner. In addition, the room is no longer so corridor-like.

So, I have now stepped out of my comfort zone and swapped the kitchen with the bedroom. It actually doesn't look that bad... The drawn-in furniture (except for a proper chest of drawers under the TV) in the office, living room, and bedroom we already have, and everything fits, which at least saves on the furnishing budget.

Then mirror the G and you can go directly from the garden to the kitchen.

Somehow a guest toilet is missing I’m just picturing a bunch of teenagers on Saturday evenings in the living room and I want to go to the restroom or take a bath lightly dressed...