You can make a cage around the egg site that has holes large enough for hatchlings to crawl through. If the holes are smaller, then the turtles might get stuck in there so you'd have to check frequently. The eggs take about two months to hatch but it varies with temperature, humidity, and other factors. As long as the temperatures are normal for the native range of the turtle, it should be fine. If it's abnormally cold or hot, that could effect results as could flooding or drought. The mother painted turtle has no interest in her eggs or babies and will not return. Many people keep painted turtles as pets. It's not good to take them from the wild. You can get captive born turtles and also ones that need homes. It's best to leave the wild ones in the wild. The babies need special care. Turtles under 4" are illegal because children are more apt to put smaller turtles in their mouth (small children should always be watched as they tend to get in to trouble). Sick (not healthy) turtles may have salmonella which could make your son sick. So, if you get another turtle, be sure he always washes his hands before and after contact. I have a turtle but only touch him for maintenance purposes and don't let my 2-year-old neice touch him.
My page on caring for hatchlings is at
http://www.fishpondinfo.com/turtles/hatchling.htmHere is my section on protecting outdoor turtle nests (pretty short):
http://www.fishpondinfo.com/turtles/turbreed.htm#protectI hope this information helps!