A little girl meets Santa Claus and everyone assumes he isn't really Santa Claus until a lot of people write a bunch of letters or something. Maybe they put Santa in jail? I think he rides a sleigh-float in a parade, and I'm like 90% sure the movie is in black-and-white.
Santa in his civvies.
* This movie is called Miracle on 34th Street. I was on the right island, at least.