Somehow I doubt it's the brevity that makes static electricity harmless. I've played around with battery-powered circuits that work by delivering pulses of electricity to the subject when the current through a circuit containing an inductor is interrupted (for example, the gag lighters that shock people work this way). Whatever current is flowing through the inductor gets sent through the subject for a short period of time. It's relatively safe because the peak amperage is controlled. If you hook up a resistor in series with the subject, the maximum current doesn't change, but the pulses get shorter. When you do this, the pain falls off, but the response from your muscles doesn't change as much. Eventually, especially if the contact area between the electrodes and the skin is large, you can produce involuntary muscle contractions with little or no pain.
Given that the heart is a muscle, I would think that a few seconds of current would be better than a few milliseconds if the goal was to produce pain without rendering any permanent harm. I could be wrong, but if I had to guess, I'd say the reason static electricity isn't harmful is because most of the voltage is across the air gap, not across your body. Also, the charge is entirely on your skin, and most is probably very close to the spot where you're about to touch something, so practically no current is going through your heart.
Surely it couldn't be very difficult to have a device of some sort in the circuit to control the maximum current. At the very least, couldn't he just put an appropriate fuse in the circuit, if there wasn't one already?
There's almost one order of magnitude between the current needed to cause pain and the current that's large enough to be dangerous. The fact that the area of contact with the wire is small, and a three-year old is larger than a racoon, ought to make the range of safety even broader.