Neat idea if you want to teach the kids survival of the fittest, but bad idea with koi. Too expensive and well-mannered to execute. Maybe with feeder goldfish, but if you think fish are messy, you ain't seen nothing yet. Turtles are truly filthy animals, littering their water with body fluids and body parts, which will soon begin to stink if not cleaned almost religiously.
There is a variety of turtle that looks like sliders and is a vegetarian, but hard to find. And still messy. How big is your pond? Unless it is over 1000g, I'd try to stay away from koi. Shubunkin goldfish, which are cheaper and far hardier, still grow to be 8-10 inches long, and are a lot easier to keep. They have long fins and awesome color variety, plus they change color often as they age, like koi.