Yes there's a certain area. At the end of one of those streets near where the stars & cars show was--I can't remember the name but it'll be on the guide that shows times.
There are several castmembers out there beforehand -- ask them which ranger goes where & they'll tell you where to stand to wait. Each color ranger has a different spot & different lines. Depending on the crowd you might get through 2 or 3 lines, so start with your son's favorite. (or have another family member get into another line as a placeholder)
They ride out on their car (which is LOUD - you'll hear them coming) then all scatter to their respective spots.
You'll love it. They REALLY ham it up posing for the camera.