If anything, I'd say your book is too short for that age. 3rd grade is 8-years-olds, right? At that age, 25 minutes to read out loud is closer to one chapter of a book they'd be reading.
At 8 years old, my favourite books were Sans Famille, White Fang and Narnia (all of it, except for the last book, which my parents decided was inappropriate). King Matt the First, The Wizard of Oz (with all the sequels) and Marry Poppins (all of them, again) were already behind me. My Nephew, who's 9 now, has already finished the first four Harry Potters, after which the content became too adult for him - the length was just fine. So there's your sampling of length you can be aiming for.
A book that can be read from start to finish in 25 minutes sounds to me like something for 4-years-olds, but even for them you don't have to limit yourself to something that short. Winnie the Pooh, which @ArcanistLupus mentions, or Peter Pan, are also quite appropriate for that age.