Rookie, I did the exact same thing you're thinking about, but it didn't work. I drilled a 3/16" hole on both sides of the receiver (with the plug in as far as possible) as close to the lip as possible, and put a 3/16x2 3/4" bolt through, putting a small nut on the end. Not impossible to remove, but a kid won't be carrying two 3/16" wrenches with them, right? I lost the plug a week later. The problem is, the plug is too short, and the hole I put in it was only about 1/4" from the end. With a little force, you could pry it loose.
I bought a third plug, and this time put about five layers of electrical tape around the plug. It's a real tight fit. We'll see how long that lasts.