The length and complexity of the story. Simple, short stories with repetitive language work best for young EFL learners.
Does the book look overwhelming? Type that is too small, or too many words on a page, can intimidate young students.
The level of vocabulary. How much of it will be review for your students? If students know less than 75% - 80% of the vocabulary, they may lose confidence in their ability to understand the story.
Illustrations should be interesting and should help students understand both the vocabulary and the story.
Finally, select a book that you think you will enjoy. It will be difficult to convince students to be enthusiastic about a story you don't like.