Lock, Shock, and Barrel are Oogie Boogie's not-so-loyal "little henchmen". They are trick-or-treaters, with faces similar to their masks. Jack sends Lock, Shock, and Barrel to "Kidnap the Sandy Claws" (the Halloween Town residents' mistaken name for Santa Claus).
You are looking for a small locktop box, big enough for small swaps and trackables, which contains a log, pencil and some small swaps. The location is wheelchair/pushchair friendly, but not so much in the wet weather and assistance may be required to retrieve the cache.