Judy Blume 
One in the Middle Is the Green Kangaroo [EPUB ebook] 

Freddy Dissel has two problems. One is his older brother, Mike. The other is his younger sister, Ellen. That leaves Freddy in the middle, feeling like the peanut butter part of a sandwich, squeezed between two pieces of bread like a great big middle nothing. So when Freddy hears about the school play, he knows its his chance to shineeven if the play is being put on by the big kids, and even if Mike says that everybody can jump. But nobody can jump quite as well as Freddy, which makes him the perfect Green Kangarooand the star of the show!