V.V.
Is it a 24 hour, continuous EEG or just a quick one? If it's the latter, it will probably take between 1-2 hours, depending on how cooperative your kiddo is. What takes the longest is placing the electrodes on the head.
The technician will "map" your kiddo's head, and may draw on him/make little dots with a wax pencil. This is to ensure that the electrodes go in the right places to give the best reading. Then they'll stick the electrodes on, usually with a little bit of goopey stuff, and wrap his whole head in gauze, to help them stay on.
And then the EEG can start! They will have different sounds and lights going in the room, to see what, if anything, triggers changes in the brain waves. And they are probably hoping your kiddo falls asleep, so they can see sleeping brain waves, too (hence the 5 hours of sleep).
All in all, pretty painless, just tedious.