Saw it on television mid 80’s. Color, in english.
These kids(boy and girl) go to visit the granparents possibly another family member.
There was a resaca or large creak separating the houses in the back yard. Across was a white house with an old man. The kids see some things that concern them though i cant remember what.
They end up getting kidnapped by the man and are stuck underground.
The escape scene was them sliding is some tubes like you see in a water park. White tube.
I want to say there was aliens but i am not sure.