So, a small bungalow with a gable roof. The children's rooms are rather small, storage as well, and no space for an office. How about a staircase to the attic: office and playroom plus storage. Then the roof would be insulated instead of the upper floor ceiling, the electricity would be routed up, and a large window would be installed on one gable side. So initially just preparation, the children share one room at first and the office downstairs is initially the other "children's room". And when the children grow older, you can further develop the attic, so playroom plus office or one of the children wants to move upstairs. The attic is suitable as storage from the start, if necessary just laminate or PVC flooring and a bare bulb on the ceiling. It costs you the staircase plus its space and a gable window. In return, the space required on the ground floor is smaller and therefore cheaper.
Disadvantage: your wife can't go upstairs then, but a stairlift would always be possible afterwards. Costs start at about 8,000 euros for spiral staircases.