I've had both land turtles and aquatic turtles as pets (as well as a huge variety of other reptiles). I found them easy to care for, but then, I just let them have their run of the house and yard.
Remember that you can catch salmonella from turtles, so if the kid isn't good about washing his hands, you might run into trouble there.
Oh, and the aquatic turtles are a lot more care-intesive. I would not recommend them as a pet for anyone who is not an adult enthusiast.