12yrs old? A Kindle Fire????
Not due for another month, and I suspect it won't come with the regular Market. Probably Amazon Market. And a root process won't probably be available for another 2 months or more. Considering nobody's really had a hands on with it, it may really suck.
For the younger one, a Fire might be ok, provided they release it on time.
Also, you may want to check out the HTC Flyer, or T-Mobile's Springboard which will be released soon. Specs and hands on are good, but you have to have a 2yr contract I suppose for the springboard.
For the older, I would be looking at something he or she could use for the next 2 or 3 years. So a full size is warranted. Acer A501 or A500 would be my choice

Extremely easy to root, and has a full size USB2.0 to connect periphials. A500 16gb (expandable with SD) was currently only 340$ last time I looked.