I used to have 4 turtles, but I gave them away, in order to become a Merchant Mariner. Understand, that since the early 1970's, it has been illegal to sell or own a turtle with an under 4 inch carapace length. Also understand that most easily available turtles will grow quite large, don't like to be touched by people, and produce TONS of uneaten food an poop. Also, if well cared for, your turtle will live between 25 and 150 or so years depending on the species. That said, turtles can be wonderful. The law was meant to stop kids from getting turtles that they would want to touch, so that kids wouldn't catch salmonella from them. A preponderance of turtles carry salmonella, and since kids are lousy about washing their hands, they often catch it from cute little turtles that are fun to touch. The only ones you can get at such a small size are usually Red Eared Sliders. They are of course sold illegally on the sidewalk in Chinatown.
The upside of the illegally small turtles, is that they are bread for the pet trade. The unintended consequence of the 4 inch law is that it was no longer worth breeding turtles for the pet trade, so now all of the turtles that are sold legally are wild-caught. Thought I might help make an informed decision. Are you sure your kid couldn't be satisfied with one of those African Aquatic frogs or something?