Ralphie and Randy are waiting in line to Santa. Right before the little girl in the aviator cap says, "I like the Tin Man", you can see Randy look off camera and smile very quickly at somebody.
I don't want to nit-pick, but since this is an ongoing argument between my husband and myself, I feel the need to point this out. The kid in line behind Ralphie is a boy, not a girl. He is wearing pants. Girls in 1940 wore dresses (especially for such an occasion as going to see Santa) and the child is listed as "Goggles" in the credits as being played by David Svoboda. So if it is a "she" then it is just another slip-up to add to the list.
Actually, I believe that was deliberate. He was looking off to the side in embarrassment at being seen with such a dork. He is probably smiling at his parents or a friend from school. And yes, the Wizard of Oz fan is a boy.
I don't think you can say that it's a slip up because there is no way to know what he is smiling at. For all we know, the director told him to smile as if smiling at his parents.