Norman, turtles are great! But, they will eat some plants and animals. My page on turtle ponds is at
http://www.fishpondinfo.com/turtles/turpond.htmI suggest Eastern painted turtles. Red-eared sliders are easiest to get but they also grow larger and like to eat more fish than painted turtles who seem to prefer insect larvae and smaller animals. Sliders also eat more plants.
If the pond is not fenced properly, the turtles will absolutely leave. It seems to be the goal in life of most turtles to "escape."
The main predators for larger turtles are raccoons and dogs. For smaller turtles, add in herons, larger turtles, and other mammals and birds. I would mostly worry about raccoons. I know on the box turtle list to which I belong, people have found their turtles with all legs and head bit off. And box turtles can hide inside their shell unlike aquatic turtles.
Whether you get painted turtles, red-eared sliders, yellow-bellied sliders, red bellied turtles, or some other turtles (you don't want snapping turtles), they will do their best to stay away from people, especially children so physical contact is not likely. If a wild turtle is grabbed, it will bite. Those used to people do not. My turtle has not bitten me but I avoid putting my hands near his mouth in case he mistakes them for some big worm. For the safety of the turtles, avoid handling them. Healthy turtles don't carry salmonella but, just in case, children should not handle turtles and adults should wash up before and after doing so.
Good luck!