My sister and I loved a movie when we were growing up in the early 90s but forgot the name.

The movie opens with an older man concluding the filming of a tv show for the day off a dock or on boat. The show includes 3 assumed puppets that are actually monster/aliens that are being held captive. A boy and girl discover that they are actually alive when they see the monsters playing cards (poker) with the old man through a peephole in the wall. The monsters are kept in a treasure chest so that they won’t escape. The children try to free them.