This animated miniseries was originally broadcast as part of the ABC Weekend Specials. It tells the story of Penny, Peter, Patsy and Pamela who go through a trap door in the floor of their playhouse in search of their brother, Paul, who's mysteriously gone missing. While down there, they discover the world of Og, populated by small, green people who can only say the word "og." This series was based on a Canadian children's literary classic of the same name by Pierre Berton.
This has been released on DVD and can be found on Amazon. Above is a little taste of the show, uploaded to YouTube by spockvideo.
Obscurity factor: 8 (available on DVD, not well remembered)