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

Why don't you choose a standard floor plan? For example Ambience 111 V3 Bien-Zenker or the one from Nordlys and Kind upstairs

The budget is a maximum of 300K €, if necessary we can take on EL in the form of wall work (puttying, priming, plastering, painting).

Including incidental building costs or without? I don't see the flat roof extension in either case. I'm rather seeing a separate glass terrace roof. I don't see a very large base area - I see an attic conversion! That's where the price-performance ratio is still manageable! Take a look at Scanwood and Danwood and their houses.

Better?
On the upper floor, the children's room, a children's bathroom, and a storage room are to be created?

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

But did you notice that there are also normal single beds shown in the post I linked? Neither my 2-year-old nor the 6-year-old sleeps in a baby bed. The older one has a regular wardrobe. The little one also has one based on space requirements.
What the teenagers no longer have are doll prams, dollhouses, play kitchens, Lego bins, etc. In other words, a lot of space is freed up that can be used differently. The floor is then only needed for laundry storage.
But you can comfortably live your childhood in 11/12 sqm without feeling like a sardine. A bit of creativity in furniture selection is required. Whoever can and wants to give their child more, of course, can do that.

Including incidental building costs or without

You generally do not answer questions?

Better? Upstairs the children's room, a children's bathroom and a storage room are to be created?

Here I would quite cheekily remove the kitchen walls and the wall of the dining room to create some spaciousness. Either put a wall in the hallway up to the stairs or not, doors in the hallway or bathroom door convey and "protect" the front area, thus creating a small hallway, swap dressing room and bedroom. To make it even more complicated - sorry - consider whether to align the house along the parallel of the south side of the plot. The entrance nicely in the north, full width to the south…