Ground floor approx. 100 sqm, upper floor expandable (planned bathroom, 2 children's rooms, 1 storage room)

I don't find the matter unsolvable or the rooms too small, etc. However, the missing measurements and expansion planning are definitely suspicious. But keep your head up. We're happy to help. The more absurd the specifications, the more interesting it becomes. I only say "Eiche".

There is one room too many. The planning for the upper floor should have been done earlier. The ground floor is already in place. I would plan 2 children's rooms, 1 bathroom, and 1 storage room upstairs. Who sleeps where and when will then be shown by the daily routine with the child/children. Not every family is the same, not every child is the same. Parents' bedroom now downstairs. Where child 1 sleeps still needs to be determined. Maybe add the small room to the bedroom after all? Or a 140 bed and a crib.

Ok ok, understood that we have to make compromises.

Then this floor plan seems to fit best. The 1m and 2m lines would still need to move outwards a bit, which would give the rooms a little more space.

Now, based on Katja's designs...
Although without looking, the hallway turned out the way I remember Katja's designs
With the dimensions from the OP

The storage room is removed and the room in the north is still flexible either as bedroom/ office/ storage.

In my opinion, the children's rooms are big enough!?

Hehe, the bathtub has already cracked too....

Huh, what do you mean by that?

Man, man, man... There is still no child yet. And when this expansion will be carried out is written in the stars. Once you have moved in, you will be thinking of other things, certainly not the above-mentioned expansion. And then, if God wills, the first child will come and be housed next to the living room. Even then, there is no need for the attic expansion – even if you imagine it differently. In this respect, if there are ever 2 kids, they will certainly survive if mom does not live next door, but 5 meters away. Your house will never become that big. The house suffers from a lot of back and forth... there will always somehow be a compromise lurking somewhere.