I can understand both sides. If you first build the house and then plan for children, it can put a lot of pressure on you to see the empty children's rooms. You never know if it will actually work out.
If you build the house when the children are already there, you know that you need children's rooms, but you may no longer have the time or only at the expense of the children to take care of proper planning and selection.
When we started planning our house (on the second attempt), I got pregnant. The house is exactly as old as our son. At the beginning of the year, we drove to the hospital on the day the construction crane for the shell construction work was delivered. Most appointments (e.g., for the kitchen) we still made while I was pregnant, including the entire electrical/KNX planning. Still, there is always a lot left to decide and choose finally. We have a very chilled child who we take with us everywhere. The little one already knows Hornbach inside out from numerous tile viewings, construction meetings, etc. The bricklayers even gave him a little something as a farewell after completing the shell because he was there so often. We currently live in a house and are essentially building our new house in the garden. So we live right next to the construction site, which makes many things easier. I also swaddled the little one a lot and always took him over and helped with the electrical work.
In the end, everything is as good as it is. Our son, the floor plan, the construction costs, the interest rates, the funding. Somehow, it was the right time for everything, even though it is sometimes exhausting now with construction and child.
In a nutshell: the smaller the child, the more self-determined you still are. Waiting until the child planning is complete before building means possibly having two children aged 3 and 0. In this constellation, I wouldn’t be able to free my mind for construction and planning. There would only be standard without sophistication (in the floor plan, kitchen, lighting planning, KNX scenarios, etc.). Therefore: build now, have children afterward, or get pregnant during the construction phase. If you (seriously justified) worry that the children's rooms remain empty, then plan an attic conversion as a reserve for children's rooms.