To stay with the house from your original post, this would be a conceivable attic floor with a utility room of about [ATTACH alt="DG3_1.png" type="full"]41944[/ATTACH]5 m2.
I would definitely give the lower room a three-meter-wide shed dormer on the right side, that would be 3 m2 extra (from the previous crawl space in the side alcove) plus another 3 significantly more usable m2 where the ceiling height instead of rising from 1 to 2 would be at least 2 m everywhere. No child can complain about that!
Upper floor as in your first plan, so with a separate study. And now the kicker: the utility room on the ground floor can be omitted, the kitchen moves down, the living room becomes bigger. Starting from the ground floor in #63, so with a toilet on the right side.
Or you move the kitchen 40 cm further up and have your study with almost 7 m2 instead of the utility room there. A better little nook, yes, but close to the activities on the ground floor. The previously planned study on the upper floor then becomes the utility room (washing, ironing, storage).
As long as child 2 and child 3 can share one of the rooms in the attic floor, the children's room on the upper floor will be your study, and later one of the later-born children will move in there, and the smaller study on the ground floor will be used by you for the first time. And if one of the children should move out, you have your study on the upper floor again, and a storage room becomes free on the ground floor.
Does that sound workable?