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

40 or 35° is too high. In the worst case, you should need 35° flow temperature when it is really cold. The underfloor heating circuits require this flow: keep the thermostats permanently open. Lower the heating curve until it is no longer too warm, then adjust the circuits. Take your time and do not change too much at once. Notes help. I cannot contribute anything regarding the setting for Vaillant. Example of a heating curve in a new building: 24° flow temperature at 10° outside temperature, 30° flow temperature at -14°.

Yes, it can be that low. I already explained to you what you have to do. Why don't you just do it?