I'm flying from Gatwick for the first time next week with a train nerd son. We are flying from the North Terminal. After we've dropped our bags (have checked in online already) can we ride the monorail to the South Terminal and back to kill time/entertain ourselves?
Yes, you can see on the terminal maps at http://www.gatwickairport.com/at-the-airport/passenger-services/airport-maps/ where the shuttle runs, you can use it after you check in but it is before security so make sure to leave enough time for your flight. I also feel the need to point out that the shuttle technicaly isn't a monorail, not to get tool technical it has rubber ties on a concrete track.
Yes, you can ride it after checking in, as long as you've not gone through security. There are no checks or controls, so anyone can freely board it and ride back and forth as much as they like (within reason - bear in mind that security is heightened at the moment, so they might stop you if they think you are acting in a suspicious manner)
Also, you don't mention which airline you are flying with, but if it's Easyjet, allow extra time to get through security and to the gate, as many of their flights go from an 'annex' which is a fairly long walk (700m according to the airport website) from the main part of the north terminal.