Quoted from minnesota13:
1. Pull the top switch out as Chris indicated.
2. Close and lock the coin door.
3. Power-up and play the game.
The safety aspect is really more for the safety of the machine so that clumsy debuggers don't short coil voltage into the switch matrix and other circuits.
Do the above and if you are concerned about the safety with your kids, pull the key out and put it in a safe place.