Children's first pets. Am I crazy or what?

So at the beginning of the year, my wife and I made a deal with our oldest daughter that if she took care of our four cats for a month that she would be able to get a pet of her own. Now that time has come and she is being a normal kid and flip flopping between what she wants. We limited it to a small pet, so no dogs, or cats because we all know how that would go. A week later, I would be taking care of it.

So we have decided to let her pick this Saturday, which coincides with us getting our tax return, and now she is still debating between two female rats,(2, I guess because they need company or something like that.) or a guinea pig.

So I turn to you my readers, those of you that have kids, have you given them the responsibility of a pet before? How old were they and how did it work out for you? What sort of pet should we let our nine year old daughter have? Also as a surprise to my youngest daughter (3) we are going to let her pick as well. Knowing her it will probably be whatever her older sister gets, but just in case it isn't, what do you think she should get?

Anyway, just a few questions and wanted to get some opinions on what worked for you.