Sorry, just got home yesterday on the red eye and had a field trip with my son all day to EPCOT, so to say I was exhausted last night is an understatement! LOL.
Okay, I only needed to show 1 key. But, I think I know why now and it may not be the same for everyone else. My husband works for Disney so I sign the others in with me for the day, so I'm guessing that's the only reason I need to show only one key!
But, I did pay attention for you while waiting in lines, and it appeared only adults and older looking kids had to show the keys, I did not see them require it to be shown for pretty much any kid in a stroller!!
Sorry, hope that helps, just realized while I was there that my situation is not the same! I did notice that it really also depends on who you get!! I saw many CM's being sticklers and some just let the kids go when just a parent showed the key, so I'd say it's also just random sometimes as well!