My impression is that these things are done to avoid censorship in Germany, since German law doesn't allow any Nazi imagery in children's programming, or is it in movies/TV altogether?
They don't allow Nazi imagery for shits and giggles but it's allowed for educational purposes, in art or reporting (which means random people waving a Nazi flag is illegal, the news filming that and showing it on tv is allowed).
If Nazi's are portrayed in a movie using the Swastika etc. is fine, it happens all the time. And it's not just limited to period pieces taking place in nazi Germany, it also applies to Neo-Nazis. Take American History X for example, the Nazi imagery was justified and no one had a problem with it.
Obviously the context also matters, you're not allowed to glorify nazis or deny their crimes, so someone making a movie that does that cannot use the imagery without being punished because it's obviously not educational and you can't argue "art" because it falls under hate speech.
There are no special rules regarding children's programming that I'm aware of, in fact all children are exposed to Nazi symbols in school, learning about it is part of the curriculum and seeing the pictures of the Nazi's crimes is important, so it's not like they have no idea what the stuff looks like.