I think this guy (Bob Meyer) and jjbrain above have the best advice so far. Here's my opinion as a mom of kids from age 13 down to 4.
First - you don't want to pay a monthly fee to a mobile provider for an 8 year old - stick with something that's just wifi access - it's all you'll really need, as long as you already have wifi in your home.
My 13 year old son has had a Kindle Fire since last xmas (so, a year). It IS, essentially, a tablet that also acts as an ereader. He does a lot on it, plays a lot of games, reads a lot of books, can research on the internet. There's also a LOT of free content on Amazon. Kindle Fire also has an app/setting called Free Time - it's for parental controls for kids & limits access to internet sites, time to play, etc. We've found a lot of educational/learning games for his little brothers - many for free or just a couple bucks.
Amazon has the new Kindle Fire on sale now starting at $140 (not the higher prices noted by others in this thread), which is a good deal and LOTS less than an ipad. However, depending on how careful & responsible your kid is, these things are NOT tough - you'd need to also get a sturdy protective case to protect it. (likely so with most tablets)
The plain old, regular ereaders that don't use apps are not something I'd recommend for a kid/preteen - it's really only for reading. I have a Kindle Paperwhite - They're great for what they do, and the battery usually lasts FOREVER, but it's no comparison to a tablet.
In our home, we mostly use Android - Android phones, Kindle fire, etc. So, an Android-based tablet is a comfortable fit. We can share some apps between devices, no new systems to learn, etc.
But - if you already have an iPhone, then you already are very linked-in with iTunes, and an iPad might be your best choice. My 2-year-old niece uses an iPad, and plays a lot of games on it. (my opinion - she's WAY TOO YOUNG to be on it so much, just don't tell my sister I said so!)
<div>
I just think the iPad is pretty expensive in comparison to alternatives and comparative versatility, especially if you aren't already in the Apple/iTunes universe.
There are also many cheap Android-based tablets, even some built for kids (extra sturdy & often specialized content). Most Android-based tablets access Google Play, and there's a lot of free or cheap content to be found there. I think that the Leap Frog Company may also make a pretty decent tablet for kids, but might be a bit juvenile for an 8 year old.
Based on your finances and feelings about conservation, it might not be a bad idea to get your 8-year-old a "starter" tablet that's built for kids & has built-in parental controls & safeguards. One he/she can learn on, earn how to use & play on, but won't easily destroy or find her/his way onto inappropriate websites.
Then, in a year or 2, upgrade to whatever tablet is available then & works best with your lifestyle.
Personally, my 7-year-old would love to get his own Kindle Fire, and he may just get one for Xmas, but WITH a really tough case. He's already more responsible than my 13-year-old, who's cracked his Kindle's screen.
So, I think the choice has a lot to do with which system (Apple, Android, etc) you already use, and how careful/responsible your kid is, and if you want apps & internet access.
Good luck!!
</div>