I'd recommend rats over mice. I found it really difficult handling mice.. I'd imagine it'd be pretty tricky for a 7 year old too!

It's really, really, really important that you get two or more. They're very social creatures. Even if you could spend hours with them every day, there's no subsituting the grooming, playing and interaction another rat/mouse can provide.

Rats are just wonderful pets though. They're incredibly intelligent and bond easily to their owners. I think of them as little dogs because of their level of intelligence actually.