The Power Rangers come out at certain times at MGM (you can check the board or pick up a sheet with character meetings along with your map).
They are at the far end of the New York Street (right by Lights, Motors, Action! Extreme Stunt Show). They come out on their car, blasting the music from the show. Each ranger stands at a specific spot so make sure you get there early and get in line for you son's favorite (the Red Ranger usually has the longest line whenever I saw them). Depending on the weather and crowd they will stay out for a while. When it is time for them to go they will make a five minute (or so) announcement.
It is actaully very cool to see because the kids and adults get to into it.