I agree that tortoises and land turtles are not for beginners, especially a child. I realise, and you do too, that the turtle will be under your care. They have a lot of needs and high maintenance - diet in particular.
If you are adamant on getting a turtle, I would suggest you stay away from tortoise, box turtles and other land based turtles or even semi-aquatics. In my opinion, aquatic turtles, like sliders, cooters, maps and painted turtles are easier to care for when it comes to health and feeding. The thing is you have to have a tank and a nice expensive filter to make the hobby enjoyable. But as I said, common aquatics are easy to care for. The water changes are a pain. If you have a small tank and especially a small cheap filter, you will dread cleaning the tank as it dirties every other day. With a good canister filter, you just need to do water changes every week or two and clean the tank once a month or so.
I recommend a southern painted turtle. It stays under 5" when full grown, easy to care for. It likes to bask and it's friendly. It will beg for food and swim around. Stay away from various sliders anc cooters which get to 7-14" depending on the species and sex. You can also look into Mud and Musk turtles, which stay small 5-6", but are very aquatic and are reluctant baskers.
As for semi-aquatics, I think of spotted turtles, wood turtles (NA and CA), various Asian box turtles, which are trickier than the previously mentioned aquatic turtles.
BTW, I assume you are American, that is why I am mentioning some turtles that are illegal to keep or rare in Ontario, Canada. As mentioned before, turtles are best to be observed pets, not to be handled regularly. And nobody has mentioned the "S-word." Some turtles may carry salmonella and that is why it is good practice to wash your hands with soap after handling all turtles or any other animal for that fact.
Cheers,
JJ
Cheers,
JJ