Gina Bellisario 
Super Pet-Sitter [PDF ebook] 

Ellie Ultra has a lot of responsibilities as a superhero. So when the chance to become Hamster Helper for the class’s hamster, Squeak, comes around, Ellie isn’t so sure she wants to add another thing to her list. But her name is picked and now it’s up to Ellie to make sure Squeak is taken care of. All is fine until a mishap in the lab happens and suddenly Ellie’s not so big any more–literally! A tousle with the Ultra Shrinker makes Ellie three-inches-tall. Now she has to take care of Squeak AND grow herself big again before it’s too late! And as Ellie soon finds out–it’s not easy being small.