There are disconnects that work using an electrically-held 'contactor', however they are quite expensive for residential use.
Primarily, they're used to turn off some kind of motor or appliance, or even an entire panel in the event that someone pushes a button. They're generally configured so that while in the 'on' position, they apply the voltage needed to close the connection, allowing power to flow. Hence, if the control circuit itself loses power, it still does its job, perhaps inconveniently.
The actual buttons can operate on anything between 24V and up, depending on the configuration, and can be a key switch (though, that stops it from being considered an emergency disconnect)
You can check at your local supply house, both Square-D and Cutler Hammer make a variety of them from 30 amp / single phase ratings on up to really big stuff. Basically, your key switch is low voltage that controls a relay. The relay controls a much bigger contactor (also basically just a relay) which actually controls power to the panel.
If all you need is the ability to keep kids out, and there's no need to expand the setup to shut off your shop in the case of emergency, then go with what Tester101 suggested. And, if you're getting into control circuits (however basic) and are even a little uncomfortable, get an electrician out to help.
You can also just get a contactor rated for the amperage of your panel and a suitable listed and approved junction box, but ... don't go doing that unless you're very sure of what you're doing. Get the made to purpose one with the enclosure, ground bar, insulators for the ground bar, etc.