The Witches (1990)

"Saving the world from witches is a tall order for a boy they've turned into a mouse!"

While staying at a hotel in England with his grandmother, Helga, a young boy named Luke inadvertently spies on a convention of witches. The Grand High Witch reveals a plan to turn all children into mice via a magical formula. When they discover the eavesdropper, the witches test the formula on him. Now, with the help of Helga and hotel manager Mr Stringer, Luke the mouse must fight back against the witches.