The children's room is too warm in the new building.

Sure, you just shouldn't be surprised why the flow temperature is higher than with others. Therefore, it is relevant to know

That is not completely settled yet. And it doesn’t need to be optimized down to the thousandth. It’s a gas boiler, so every degree is not crucial. Edit: only now read the other posts. Well, it does have to be comfortable. In my old building, with moderately insulated façade, windows partly from '92, I had a flow temperature of 35 degrees this morning at an outside temperature of 2 degrees. Room temperature everywhere 22 degrees.

You are exaggerating. With this argument, one could just as well keep their fingers completely away because it is warm. At what point is the optimization sufficient?

If the individual needs are met and the costs remain OK. With a gas boiler, it is important that it does not cycle too much and that a reasonable return temperature is reached. That way, the condensing effect works best. Or am I mistaken?

You are on the right track. Just one note:

I had to raise the heating curve to 0.5 yesterday because my wife complained that the house was not getting warm enough. At the current temperatures, I am then at a flow temperature of about 32 degrees.

I don’t know the settings for Vaillant, but 0.5 usually only represents the slope, that is, how strongly it reacts to the outside temperature. I can also shift the heating curve up or down without changing the slope.

If it is too cold at 10° outside temperature, then the minimum temperature should be increased first, not the slope. Otherwise, it will become too warm when it gets colder outside. If you haven’t done so yet, it’s best to study the manual.

This is my status for your settings:
Start: 25°
End: 35°
Slope: 0.5

I have one more question. Currently, the maximum flow temperature is limited to 35 degrees. With the large installation spacing, I could imagine that this might be too low if it gets -10 degrees outside. Should the flow temperature never exceed 35 degrees, or is it allowed to be a bit higher? I ask because I currently have a flow temperature of about 32 degrees. However, the living room is kept at 22 degrees all day. The thermostat only switches off at 22.1 degrees. Therefore, the circuit remains open all day. Is that okay? This means that I couldn't heat any warmer via the thermostat even if I wanted to.