in which 3 children's rooms, a bedroom, and a bathroom fit and the knee wall is a maximum of 50 cm. Is there such a tricky floor plan?
Plan it yourself – nothing will be spoon-fed. Three children already clearly deviate from the "standard" nowadays, so the third child will only be planned as a "guest" in the standard design. And knee walls under 1 m are not in demand either.
Unfortunately, you can't just rewrite a design with a 1 m knee wall to 50 cm: in the bathroom with a dormer, so you can stand up from the toilet without hitting your head, that's still doable. But in the bedrooms, this means you can't place a bed against a wall. That means the rooms are effectively narrowed by half a meter in usable area.
So you would practically have to look at houses around 170 sqm if, after the "reduction," you want to end up with about 140 sqm usable – even if, for example, you could increase the typical design's roof pitch from 30° to 40°.
Am I right in my gloomy suspicion that the mini knee wall is mandated by a specific development plan? – then please name its other framework conditions.