It’s another translation gone wrong. The poster was advertising a children’s cinema showing the Jungle Book and used a picture from the 1967 animated film.
The German word for jungle is “Dschungel”, the word for book is “Buch”. Hence the film is called “Das Dschungelbuch”.
Unfortunately it somehow became translated as “Dschungelbub” – “Bub” being the word for a young boy.
Well, I guess it fits in with the film…