The Witches (1990) on Netflix

Recently orphaned Luke Eveshim spends a holiday in a hotel with his grandmother Helga, who taught him a lot about witches. He befriends fat glutton Bruno Jenkins, with whom he lurks around a convention of women. They find it's a cover for a witches convention, where the international Grand High Witch Eva Ernst instructs the 'soft' Englsh witches how to dispose of all children. They are discovered, even transformed into rodents, yet escape, and must stop the evil plan at all cost.