We watched the Charlotte's Web review and talked about how they describe the book, story, and characters, but don't give anything away about what will happen. Starting with a question and ending with a cliffhanger are also key elements. The Book Chook also has a post about writing a review for kids that has helpful steps and suggestions. There is also a section there with suggested questions to help guide kids in writing their review. This site is meant more for written reviews I believe, but would still be helpful in writing the script for a taped book trailer.
I am excited to see what the kids come up with!