Despite spending time living in Berlin and directing the movie where the actors performed completely in German, director Cate Shortland doesn't speak the language.
The derelict tank the children pass in the forest is a post-WW2 manufactured Russian T-54/55 or T-62 tank. The Balkan cross painted on the turret is indicative of an early war paint scheme. Later in the war the 'lines' were thicker.
The German "das Baby" or "the baby" is used often and is in fact a recent Anglicism, and like much of the script, a word the Germans wouldn't have used in this period.