Planning single-family house with approx. 200 m² - Opinions wanted

We didn’t want that because of the mess that is surely there.

I am myself a cis man (so born as a boy and can therefore look back quite far) and had advanced biology classes: so I can assure you that leaving socks lying around everywhere is by no means genetic. Tidiness can be learned, provided one is not "genetically damaged" in that regard.

With children, every second of sleep is worth gold. Better to endure chaos than to wake your partner. Children don’t stand immediately in the bedroom. Privacy never hurts. Those who talk about chaos being disturbing are usually tidy people.

Using the upstairs hallway as a play hallway is just putting a label on useless empty space. Playing happens downstairs and later in the rooms. Rethink your plan.

Storage planning for the kitchen is a good suggestion. Talk to friends who have children about what else is coming. As it is, the kitchen is only good for microwave meals and coffee.

Underfloor heating costs for the system we have chosen

There is nothing to reconsider. In case of doubt, I would cut pretty much everything else to afford a decent underfloor heating system. Your wallet and your nasal mucous membranes will be extremely grateful to you in the long run.

But back to the topic, the draft has a few minor flaws as others have already noted, but I think the basic idea is good.

I would omit the T in the bathroom.

Underfloor heating costs about €4,000 more with the system we chose, so it's worth considering.

More than what?

Do a storage planning and see what will be standing all over your countertop.

Yep, if I subtract 1.80 on one side for tall cabinets, there is not much space left for (toaster, bottles, kitchen machines, KVA, and just whatever you put down). Less than with us (2-person household) and with us the space is unfortunately not sufficient.

Using the hallway upstairs as a play hallway

Is it supposed to be like that? Oh dear, that's a hallway that must be used for walking through, so not a play hallway.
And is there supposed to be no underfloor heating in the house?

I actually wonder why the design was posted here if it is already “finalized”?!

The island can, if I see that correctly, only be about 235cm long. That is too small to sensibly put both the cooktop and sink on it. Therefore, the counter space in the tall cabinet row will be needed for one of the two. Maybe the kitchen could be placed in the right living area. The path from the bedroom to the bathroom is unfortunately quite long. Normally, I am not a fan of 2 doors to the bathroom, but here one could consider making the bathroom accessible from the dressing room as well. Maybe a few walls can be shifted (bedroom wall downwards towards the hallway, so there is space for wardrobes) and 2 bathrooms planned on the upper floor. I would definitely do that for 5 people.