If you are willing to do a major hack...
You can buy a button in any electronic store, such as RadioShack. These are the cheapest thing. Something like this:
You can then open the case of your desktop and find the two wires that go on its power button. Cut those wires, splice new longer ones. Have them come out the back of the desktop case and go all the way to a place where your children cannot reach. Then attach the ends of the wires to the button's contacts. You will have moved the on/off switch to a higher position while keeping the desktop case in place.
If you disconnect the wires from the motherboard, you will have to reconnect them exactly as they were in order to be able to turn it on again. Every motherboard manual has a section detailing the wiring of the on/off button - it goes along with the wiring of the reset switch, frontal USB ports, and for some old motherboards, an internal speaker.
Disclaimer: if you don't feel confortable operating a soldering iron, or if you are not skilled with electronics or electric stuff, this might not be a good idea. You may risk shorting your motherboard. Otherwise, have fun!