Hello!

I'm looking for a movie of 80's or 90's, where a young boy had a friend who using a magic formula could miniaturize.
He lived in a hole in the wall of the house. There is also a dangerous rat who was terrorizing.

Thank you for your help!

Answer found : Nils Karlsson Pyssling (1990) - IMDb (http://www.imdb.com/title/tt0100264/?ref_=fn_al_tt_1)