There are many versions to this story. The original is by E. T. A. Hoffman and it is definitely not for little children. It has been rewritten several times with the most famous depiction of it being Tchaikovsky’s ballet. The version I'm recommending is published by Walker Books. One of the highlights of this book is the beautiful pop up at the end, showing the land of sweets. The illustrations of this book are also beautiful and different from other children's books. The story is also told in a way that would appeal to children of all ages.  The original version, and the version illustrated by Maurice Sendak are for older children and the Maurice Sendak illustrations are a little creepy for little kids. There's also a Disney movie that's a more complex take, with an interesting plot twist. The movie is also more appropriate for children over the age of 10.