I have two Red Ear Sliders in a 500g pond right now. They're wonderful pets, but i wouldn't recommend them for young children unless there is DEFINITE parental supervision/assistance in tank maintenance and with caring for the turtle(s).
The posts above all provide great information, but i'll contribute what i know about aquatic turtles even though i'll probably be repeating most of what's already been said.
It is illegal to purchase aquatic turtles under 4" in shell length unless it is for scientific or educational purposes. This law was passed after a huge salmonella outbreak in the 1960's-1970's, mainly caused by infants putting these small turtles into their mouths/handling the turtles and not washing their hands thoroughly after doing so. However, it obviously isn't enforced very well, because baby turtles are still readily sold to the general public today.
Anyway... there's your short history lesson. :thumbsup: Now, here's what a proper aquatic turtle habitat should consist of:
1) 55g+ aquarium for 1 turtle. You can probably get away with having two in a 75g. Bigger=better!
2) UV light *AND* a heat lamp for basking. The UV light helps in synthesizing vitamins (especially D3), which is necessary for the turtles to metabolize calcium.
3) A good, quality filter. When i kept my turtles inside, i used Fluval in-tank filters (and they worked very well), but a canister filter is probably even better.
4) Basking area. Zoomed makes turtle basking docks
that float and attach to the side of the aquarium with suction cups. These work well for juvenile/young adult turtles, but i'm not sure about heavier adults. You may want to look into that.
5) Decorations if desired... HOWEVER, it is a lot easier to clean turtle tanks if they are bare-bottom, especially since you will likely be needing to drain most/all of the water on a weekly basis. But if you do decide that you want some gravel at the bottom, don't use anything that is small enough for a turtle to consume, as i have frequently heard of them eating small pebbles (which can obviously be deadly). You could also put in some anacharis, but they love to snack on it!
Anyway, hope this information helps... they're great animals, but they are frequently put into small, dirty aquariums as babies and are given away when they become bigger. If provided the proper living conditions, they can be wonderful pets and i highly recommend having one. However, they typically don't like to be handled much, so make sure that your son realizes that before you make your decision.