How children adapt to changes, language, climate, culture

Small children adapt to a new environment and learn a language in amazing speed.
That is the reason why the usual preparatory classes, which prepare foreign kids for regular German schools, are not offered for the first years of primary school: Placing them in normal German classes is sufficient and after just a few months they are usually up to speed in all subjects.
International schools exist in bigger cities and cost around €1000/month (per kid), plus/minus a few hundred depending on school and location. Besides the cost, the disadvantage is that your kids will remain outsiders in Germany and not have friends in the neighbourhood. Therefore they are, in my opinion, only an option if you plan to return after a short while (up to three years) and value re-integration in your home country higher.

Many thanks Beppi

Beppi provided a good post. I would just add that international schools are not only costly but most that instruct in a foreign language will offer English. Schools with other languages like French, Greek even Hebrew do exist but are less common. Unless one goes to a large city, they are not likely to find such a school. Berlin would not be a problem, Leinfelden-Echterdingen certainly would.