The Witches
(1990)

The Grand High Witch (Anjelica Huston) and all the other witches (except one) drink the soup laced with the magic potion and turn into mice, which are then killed by the hotel staff.

Luke (Jasen Fisher) and his grandmother return home, bringing with them information on every witch living in America. Luke starts to worry that, being a mouse, he won't live very long, but then the last surviving witch from the hotel arrives at their house and transforms Luke back into a human boy. She also reveals that her hands are normal and not witch-looking, which means that she's somehow turned good.

Thanks Annie!