Thanks for all the inputs, I'm amazed that I got so much feedback in so little time.
Going the circuit breaker route will mean that I have to find a lockable door on the 100A subpanel that's already installed. I don't know if that's easy or not. I'll look into that a little more.
As for Heather and Wade's concerns, I agree, and do intend to teach my children safety and respect for tools. I hope that they are interested enough to learn all sorts of practical skills for life. Even so, I'd like the added insurance that when I'm not around, a lapse in judgement won't result in loss of limb or life.