Rat Heaven by Jeanne Willis
Freddie is a pet rat. He belongs to a girl who's growing up - and growing away from the babyish things of childhood. She's already swapped her teddy bears and family days out for trendy gear and a brilliant new best friend. But what about Freddie? He used to be her best friend. How could she break her promise? And where will a little lonely rat fit into this cool new life?