Crested geckos are pretty neat. However they might be too advanced for a child to take care of on his own. I do not own any but they seem pretty simple to take care of once you have everything set up. Only thing is you have to feed them Repashy crested gecko diet or similar crested gecko diet.

Water Dragons usually don't cost a lot and are easy to care for. However set up probably won't be cheap.

I think for ease of care and ease of set up I would recommend Anoles. I don't know about where you live, but in the states they are usually pretty cheap. They are small though and may not be able to handle them unless they are super tame (they can be very fast). Their tank is set up similar to a bearded dragon. They need a basking area and UVB lighting.