What do you mean with „Germans“? In my friends group, in my son‘s football club, in school, most kids have a so called migration background, but the parents are mostly second generation and mostly assimilated. So what?
Anyway, like I said there is a good chance I am wrong about it Germany in general. But I'd argue that in Berlin that is pretty much the case, regardless of how large the "transient" people are compared to "longer-term settlers". Once people start having kids they stop hopping around, so I'd guess that the amount of "non-native" children that dominate the schools have immigrant parents who are mostly settled.