Iphone 3G used a pistachio with a copper wire wrapped around it for a CPU and that's going to be your main roadblock here. Each successive version of ios has placed greater demands on the hardware. Have you seen ios7 running on an iphone 4s? Not only that, the later versions of ios aren't compatible with the 3G , no matter what you do, because of hardware limitations. Modern apps are designed to work with the 4S or later so they count on a certain amount of processing power and resources. With a 3G it just isn't going to be there.
3G is good for phone calls, texts, calculator, email, and that's about it. There's nothing you can do to bring it up to date. It's too old. AFAIK Appstore didn't even exist until iPhone 3GS. The 3G just doesnt have the horsepower to handle 99% of the apps in the appstore today. Besides, those apps are designed for the high res screens from the iPhone 4 >>.
See if you can track down a cheap iPad 2 and put that in an Otter case and give it to the kid. Much easier to work with a large tablet than a small phone when you have limited motor control. iPad 2 is fast enough to be compatible with all the kids stuff you will find in the app store.
