Floor plan single-family house 240m² on a gentle slope

I thought so too. I was just not sure if the OP wanted it that way.

I would arrange the bathroom differently. First of all, there is a family with children.

Do you have a rough idea about that, meaning in terms of a different arrangement?

I have the impression that the upper floor could be sufficient for the two kids + children's bathroom. For that, I would move the master bedroom downstairs and the guest WC could be added to the bathroom.

That would no longer correspond to our room program. The parents' bedroom was/is upstairs. In the home office, 2 adults should be able to work - possibly at the same time. Adding the guest WC to the bathroom? I'm not sure if I understand the idea correctly?

That no longer corresponds to our space program.

How is it then?

The eaves heights are requirements and restrict the attic quite a bit, which is why the children's area in the attic was discarded.

When I read this, was it once planned that way? The further planning in the basement also just somehow evolved, as far as I remember. I would actually think it over properly again and sort rooms as well as consider whether a granny flat should/must exist and for whom and when. Accordingly, check accesses again (e.g. in the basement).

I would plan the whole house differently - except for the seating area in the kitchen.

I don’t think it fits. 2 offices, 2 decent children’s rooms, 1 guest bathroom by the guest room, the dark row of closets out of the bedroom.
Upstairs 4 rooms and a bathroom that are now used as dressing room, bedroom, Child1 and if applicable Child2. Later the master suite moves down to the ground floor in the area that currently houses the guest room, office and 1 bathroom. Cooking and eating feels uncomfortably large.

That no longer corresponds to our space plan. That was/is parents upstairs.

Why? Is there a reason for that? I think, for example, that schoolchildren need much more quiet than the parents. If the TV is running next door or maybe there are guests, it's hard to sleep.

Attach the guest toilet to the bathroom? I'm not sure if I understand the idea correctly?

Yes. You have 2 toilets right next to each other in a very small space there. In addition, the guest toilet door is very awkwardly close to the dining table or kitchen. That is unpleasant for everyone involved. If you attach the toilet to the small bathroom, it becomes a larger shower bathroom, which is good to use as a parents' bathroom and where a guest can also retreat quietly once in a while. If that is not an option, I would rather install a guest toilet in the basement than next to my kitchen.