My suggestions are:
1.) I would consider whether you might want to make a closed staircase.
The space underneath can be well used as a storage room, because the utility room is a bit small for the planned home technology.
Additionally, you can store rarely used jackets, shoes, etc. there very well. :-)
2.) The bay window is typical for a Frisian 3-gable house.
Ultimately, it’s a matter of taste, because it doesn’t have much practical use.
3.) Whether you need 4 or 5 rooms is up to you.
Otherwise, I also find the downstairs a bit tight.
My idea would be: remove the guest room, and instead continue the living room wall straight. Move the guest WC to where the guest room door is now.
Enlarge the utility room so that it covers the current WC and the hallway area in front of it, making it 3 sqm larger, because you will need that room.
4.) The bedroom is way too big! You only spend about 8 hours (?) a day there, and sleeping, whereas your children spend about 16-20 hours/day there, much of it awake time.
12-13 sqm for children’s rooms of older kids is, in my opinion, too small these days: That is my personal view.
Possibly an (interior?) sliding door between the living room and kitchen would be useful: That way the kitchen is always open but can be closed off in case of strong odors.
You can feel free to send me the construction company as a private message, as I know all Oldenburg construction companies very well and know something about almost every one, even though I already have a suspicion who yours is. ;-)