What is the main problem in book Witches by Roald Dahl?

The Witches by Roald Dahl mainly revolves around the problem of witches being disguised as ordinary women and posing a threat to children. The story follows the adventures of a young boy named Luke, who encounters a group of witches led by the Grand High Witch. The witches are attempting to turn all children into mice using their magical powers. Luke must find a way to stop them before they succeed in their evil plan.