I would already be satisfied with being better advised than by this lady who does the calculation (after all, authorized signatory at a larger engineering office).
It is a new building, a terraced house with a flat roof. Children's room 1 has only one south-facing exterior wall, children's room 2 faces south and east. The bathroom has a north-facing window. There are no roof windows. Thus, only the points of roof insulation or calculation errors are considered. Additionally, there is the central exhaust system with window rebate ventilators, although these cannot explain the large difference between the children's rooms on the upper floor and the living area on the ground floor. Possibly I can get the room-specific heating load directly from the developer. The planner does not want to disclose it.
To approach the topic: The guest room on the ground floor (radiator length 83m, valve opening 15cm) has a size of 10 sqm and two exterior walls, children's room 1 on the upper floor (2 radiators with 132m and 138m, valve opening 5cm) has a size of 12.7 sqm, only one short exterior wall, window area is similar.