I saw this as a kid in the early 80s, but who knows when it was made…
Basically there is a “haunted” house, some kids explore it, and find a secret room in the basement with a dead body in it. Then they get trapped, but the room is under the yard and people dig them out.
sounds similar to this: http://irememberthismovie.com/afterschool-special-type-thing-about-teens-who-discover-a-secret-room-in-their-house/