Floor plan single-family house approx. 180 sqm

Is it even possible to build the draft without additional columns in the living room?

Now that you mention it. The short wall sections in the living/dining area, large windows, and spans do indeed look ambitious. The structural engineer will probably have a say in this. How are the loads supposed to be transferred there? Already substantial lintels are necessary, and due to the small wall sections, these walls are practically executed purely as supports. If this is technically feasible at all, I see the euros just flying away for the reinforcing steel.

The OP seems to have forgotten their own question…

Then to the actual question:

We are very satisfied with everything except for the fact that the Child II room is too narrow for us.

The depth in the entrance area (Child II) is about 2.5 m. The depth in the lower area is about 2.37 m. For comparison: Child I approx. 3.58 m x 5.0 m

The children's room 2 has over 18m² of space. There is no need to save anything; it fits well into the style of the rest of the house. A double bed also fits easily into the "niche." For teenagers, it really does not matter whether you get into bed from the foot end.

Do you even have the money for such a big place? Sure, you can plan 22m² for the bedroom and then put in "only" a 4m² bed. Or a 14m² tube bathroom, which could also be done in 10m². Etc.

Yes. The floor plan is not really successful in terms of dimensions. A lot of unnecessary space wasted for a lot of money. The house can easily be built with 160 m2 and without unnecessary hallways/corners and still achieve almost the same result.

Is it even possible to build the draft without additional columns in the living room?

At least with lightweight walls on the upper floor, yes.