I think you made a good decision in saying no. As much as I hated my parents for saying no to a dog for over 12 years, I now know that it was an excellent decision. I was far better prepared at the age I got Kai at.

Ratties need a lot of attention as they're social animals. I think of it as having a very small, compact dog. Maybe it would be better to wait until she is in a higher grade (after she begins highschool). Things will only get tougher as she gets older.

As for the responsibility thing, how about getting her to scoop the kitty litter and feeding the kitties as a "pre-rat requirement"? That way, she can get a taste of what it's like to own her own pets.