I'm not even sure if kids enjoy kindles to be honest. I always see people on the BART or bus's with Kindles.
I have 10 year old and 13 year old, they both like their kindle fire. We also have available for their use a 10" touchpad running andoid, and a couple ipads. One of them also has an ipod touch and the other has a wifi only iphone 3gs hand me down.
Personally I am not too thrilled with the fire and really don't understand why it isn't better integrated into the Amazon ecosystem with cloud based categories and better segmentation of books from a shared family library. But they like the size and ability to use it for reading and games, they take them everywhere.
The fire tablets were purchased for $149 last year before Christmas, perhaps they would like the nexus 7, but I think they actually like the simplified interface on the fire and the fact that I keep adding books I get free from eReader IQ which show up on the carousel...