Bungalow floor plan for 3 persons, 130 sqm, please opinions...

I attached a picture of various houses that inspired us.

You picked the right ones straight away, for the small budget. :D

The children permanently stay at 90 cm in height and never need a proper wardrobe or even a bed.
Oh man, what you can (have to) read here...

I also didn't have a huge children's room as a kid, and 12 sqm is standard in apartments. I really don't see the big problem. A child doesn't become more loving or educated just because they have more "space" to live in.
When I think of my friends as kids who lived in the panel building, they had at most 12 sqm for two people.
I also think 12 sqm is enough for a teenager.
A shared flat room as a student is not always bigger either.

Because the door is located in the east under a canopy, your kitchen is very dark. You will almost always have the light on there.

The door would be in the south. The east is on the street side. Sorry, it probably got a bit lost due to the regular north orientation that everyone recommends.

You picked the right ones for the small budget right away. :D

Yes, that's true, but I believe you can get more out of less. Sure, it won't be as lavish, but you can get ideas, like the clad exterior wall.

the terrain is flat, only drops a total of 3 meters to the west. I honestly don’t know what other information I can provide!?

For example, the dimensions of the plot and budget expectations. It doesn’t help if someone designs a low-budget house for you here, but you could and want to spend much more. The other way around as well: features in the floor plan are expensive – what am I supposed to suggest huge sightlines that take up several sqm and you can’t afford?!

one idea would still be to expand the main house with a pitched roof upstairs for children’s rooms and a children’s bathroom, but keep the rest of the rooms on the ground floor. The thought behind it is whether the space for a large staircase is worth it

That’s already a good approach: the child has an adequate room on the ground floor for a few years, the office goes to the attic (flat pitched roof), and when they can and want to sleep alone, you switch. An average staircase is also sufficient, e.g. steel stringer, it’s quite modern, timeless, and cheap. With that, you have good use of the roof, which is there anyway. This way you can minimize downstairs a bit and balance financially – and you have the spatial separation. Later you have a nice room upstairs for the next hobbies (yoga, guests, etc.) What about an architect? I’ll check at home what I still have saved on the computer – if the plot width is known… It’s also just that we have summer weather right now, so I don’t stay in front of the PC – but sketching on the lounger works ;)

You have to be able to afford large rooms. Small children's rooms are okay with a small floor area. Only with 200+x sqm, a parent’s area of 50 sqm, and a large gallery are 12 sqm children's rooms too small.

Here the office is too small, child and parents swap, kitchen too dark