Single-family house floor plan approx. 190m² - Optimizations

The OP does not have two children's rooms above, but a study and a children's room - which makes sense if there is only one child.

Ha, I completely overlooked that. What kind of job requires a bed in the office? Erotic actor? Yes, I think the OP really needs to reconsider exactly what he needs and would like. If the beloved little prince remains the only child, then maybe he can have a bedroom and an office himself. Bed and clothes in one, computer and backpack next door - maybe connected by a passage. Or he gets a children's bathroom - very popular with girls. Guest and office - the room downstairs is then more than enough for both.

That would be, for example, a quickly scribbled solution with a "children's suite" if the work area moves downstairs:
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Bedroom with its own entrance, next to it a separate dressing room, and if desired, still a door into the bedroom.
Instead of the narrow corridor, a children's bathroom, which would then be accessible from the children's room. You could also make that into a utility room, of course then accessible from the hallway.

The rest is a spacious children's suite with a somewhat separated sleeping area.

Windows would still need to be adjusted a bit - but surely a solution can be found.

You could also consider making a utility room and a children's bathroom and reduce the size of the children's suite accordingly. So after the bathroom a small utility room for washing machine and dryer, then a small children's bathroom, the rest is still certainly large enough for a children's room.
But then the child would no longer have access to the roof terrace. The question is whether that makes any sense at all if there is a garden downstairs. Sure, it can be great for a teenager, but always having to carry every soda up from downstairs? No lazy teenager likes that. I predict: the roof terrace will cost a fortune and will fall into disrepair in the future. Better a green roof, where you can make a door from a utility room to put out a drying rack now and then, and that’s it (also saves you the railing for such a terrace).

The question is whether the [Dachterrasse] makes any sense at all,

I would kill that one first. Who is supposed to sit there?

Where was north actually? Nothing is marked on the plans.

Overall, this way the child has a nice suite, the parents always have to go to the bathroom across the hallway. Considering that the child will eventually move out, but the parents will stay, I would generally prefer a solution the other way around.
But it also depends on where north is. I would leave the bathroom, utility room, and children's bathroom on that side because below are the WC and kitchen - but upstairs you could perhaps still have a little room for playing musical chairs.
However, I always find a bedroom in the south and children's rooms in the north suboptimal.

Where was north actually? Nothing is marked on the plans.

You do have the building plot section. It is oriented north. And there is only 1 plot with 680m². So it is really a gem with a southwest orientation.

I look at many plans and location plans here and at some point just don’t feel like constantly figuring out which location plan belongs to which house plan - a small north arrow makes things so much easier. I don’t think it’s too much to ask that when you’re seeking advice, people at least make the effort to scribble in a north arrow.

off topic but just my 2 Cents