They had masks made a while back to see if you could fool a security guard like in a movie by wearing it.
A couple of kids came in, fans, wanted a posed picture with them, they agreed.
Probably kids of the crew.
Edit A few people have confirmed that Adam and Jamie actually have a running gag in their live shows where they dress kids from the audience up. Myth Busted
Fuck Batman. I’m going to be Savage for halloween.