Ok, a lot of questions there! I'll try to answer most of them.
A water heater is essential for any turtle tank. You'll want 3-5W per gallon of water (so a 50 gallon tank should have at least a 150W heater).
Sand should be OK, there are lots of owners here who use sand. If you do decide to put rocks in, make sure that all the rocks are BIGGER than their heads. Turtles can and will eat rocks, and can become impacted.
Baby turtles are very fragile, and catch a chill and get sick very quickly. At 2", your turtles are pretty small to be handled that much. I'm not sure how much handling to recommend (hopefully someone else can recommend something). Hygiene is very important (and it sounds like you're already doing a good job teaching your son that!). Turtles do carry salmonella. I've heard one person compare handling your turtle like handling raw chicken. Make sure you wash thoroughly anywhere the turtle touched (hands, floor, etc.). All I can recommend is to make sure your son doesn't put the turtle near his mouth (so no kisses), or put his hands near his mouth after handling the turtle.
At 2", your turtles are a bit too small to take outside. They're a lot quicker than you may realize, and being so small they can disappear in the blink of an eye. Since they're smaller, their body temperature will also change much faster than a larger turtle, so they can get overheated or chilled very quickly. It's recommended that you don't take them outside until they're at least 4".
One extra thing, you mentioned your son likes to feed them. Turtles are opportunistic eaters, and eat until they throw up (and then eat some more). So make sure they are only getting a headfull of pellets once a day, and put the food container somewhere that your son can't get them. Too much pellets or treats can cause health problems. You can get him to help you prepare some veggies for the turtles and he can feed them those!
