So, let's assume a two-family house…
In combination with a double garage
… then choosing a shared double garage is already the worse choice.
Since I can’t really expand in width anymore, I would have to make the house bigger to the south in order to have another children's room, but then the rooms are long and narrow.
You are thinking too much in terms of "lining up rooms"
so that I have the garden in the south
The garden is wherever a garden is laid out.
The living room is also on the southwest side, where you can expect sun and daylight during leisure time.
Therefore, the patio does not have to be there.
But if I put the front door in the middle in front of the house, then the visitor who rings the doorbell is already standing on my terrace, which of course should also be in the south.
That’s what planning is for, e.g. garage or carport as a room divider, hedge as a privacy screen, or a building structure that isn’t just a plain rectangle.
Building line
That is probably the building boundary? Where is the garage or the parking spaces allowed to be? What does the development plan say about that?
that is also unfair, if it’s even possible at all.
That’s what development plans are for. That is (usually) regulated fairly.
and I have two gardens,
see above. A garden is where the garden is laid out. It can be anywhere, which is usually desired. One place where you could spend the summer days, then possibly a play area. And you said that the ground floor might be for later. Your argument with your wife and the children counts less there.
You are also welcome to make suggestions or sketches on how to place a house with an adjacent garage there.
How, if everything for you is about now and here and in an and-mode?! There are nice apartments with two balconies, facing the street and to the back. In principle: the kitchen on the ground floor is an absolute misplanning and a disruptive factor for the floor plan. So don’t insist too much on your current planning.