The Witches by Roald Dahl

 This story is about a young orphaned boy lived with his grandmother. He soon discovers that witches, disguised as ordinary women, hate children and plan to turn them into mice using their special potion. The boy and his grandmother, who knows about witches, find themselves at a hotel where the Grand High Witch is holding a meeting to carry out their plan. After the boy is turned into a mouse, he and his grandmother figure out a plan to stop the witches, by turning them into mice and plotting to rid the world of all witches. 

I suggest it is most suitable for 6-7 year olds. Rating 10/10