I think the standard route is to buy a big-ass house which will fit all your future-kids. But, when they fly out of the nest, you are left with a house too big for you to maintain

You have no children yet, and you are planning double digits. Let's assume that means 10 children, and that there will be 2 years intervals between their births, and they will stay with you till about 20. That means it will be 40 years later when the last child flies out of the nest. Also, you might want to provide a backup solution for children who fails to become financially independent at 20.

If you are going to build a big house, I would probably try making it modular -- to build it in a way that if in the future you build two or three internal walls, you will effectively split the house into two or three independent households. Each of them with its own kitchen, bathroom, etc. (With so many people, you will need multiple bathrooms anyway. So you just have to place them strategically.) So you could later split the house, keep one of the parts, and provide other parts to some of your children -- or if they are not interested, you can rent of sell them to someone else. (Also this provides you an alternative if you later decide you actually don't want so many children.)