My ds4 loves them, too! He wore his Power Rangers tshirt, and all the rangers noticed!
Check the schedule, and go to their meeting place early. Their lines can get long, and when it's time for them to go, they go away fast!
When you get to the meeting place hopefully before the rangers, there should be a character CM already there, and he can tell you which ones are meeting in which spot. For example, "the red ranger will be on that corner, the blue on this spot", etc. Find out which is your ds' favorite, and line up there first, and plan which one to line up for next, etc. Some times you can get through them all before they leave, sometimes just a few- depends on crowds.
Have fun!