This was either a kids animated show or movie. But it was about some kids who had found in their house or in a storage shed in the back of their house an underground world where either trolls or gnomes lived. I don't remember too much about it. It played a lot on saturdays. It was in the 1980s sometime. Maybe 1985.

Sounds like ABC Weekend Specials The Secret World of Og

Thanks Quinn I think that may be it... I just watched the only short clip they had on you tube and I do remember something about things ending up missing in the house. Thank you so much!

You're very welcome.