Renovation of old building floor plan division - kitchen solution sought

Where do you want to make the terrace? In front of the children's room?

It is already there in my opinion. Therefore, it will probably stay there.
I like this option best:
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However, probably also the biggest effort. The bathroom window lintel will probably not be sufficient for the wide front door. So new lintel and cutting. Maybe you even have to ask the structural engineer. Compared to that, the other option is probably easy.

What bothers me with both is the proximity of the guest WC to the kitchen. As the saying goes: "Don't eat where you shit."
Therefore, here is my radical suggestion, which is probably less effort than one first thinks:
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Maybe the hallway is not made quite as wide and the door to the corridor is closed. Instead, the kitchen is wider. But the original poster can move that back and forth until it fits. Whether the child, however, manages with significantly less space or is supposed to manage is the question. :)

I like the radical variant, it looks good, thanks. In terms of space, I would have to discuss it with my better half to see how much she really NEEDS. Regarding the hallway and kitchen, however, the variant is very good; I just thought it through on site. One disadvantage I still see with the variant is that the guest WC vents to the terrace. When we sit on the terrace with friends in summer, everyone who goes to the bathroom has to close the window when entering and open it again after, or else create noise and odor for those present...

One disadvantage I still see with this variant is that the guest toilet vents to the terrace. When we sit on the terrace with friends in summer, everyone who goes to the bathroom has to close the window upon entering and open it after finishing. Or otherwise create noise and smell disturbance for those present...

I had already thought that too. That's a pity. Inside, I still find this variant the best. Would relocating the terrace be an option? Or can the guest toilet be moved to the other side? Are there conceivable pipes there? That would of course be even better.

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Moving the guest WC to the other side is only possible with massive effort. This beautiful old house unfortunately has only a partial basement, so drainage would be difficult. The kitchen, guest WC, and hallway are basemented. Attached for illustration is a floor plan of the basement.

so that drainage would become difficult. The kitchen, guest WC, and hallway have basements.

After the drainage or water connection, I would have asked next. Where is it? And: of course, you could also redirect everything in the garden so that they (hopefully) take the correct entrance... what about that?

No other cables from above nearby?