Discharging capacitors on a computer motherboard for safety
The highest voltage you are likely to see on the motherboard of a desktop PC is 12V.  Even if the capacitors aren't discharged, that's not going to kill anybody.  On a laptop, it could be up to 19V, which is still pretty harmless.

The PSU of a computer may have a few hundred volts when in use, but that should dissipate fairly quickly.  Leave the computers off for a few days, and it will be gone.

Beware of letting smaller children near all the bits left over after disassembling a computer.  The little backup batteries on the motherboard can be pretty nasty if swallowed, and the other components can contain all sorts of toxic chemicals.

Beware of shorting out laptop batteries, as lithium cells are notorious for catching fire