I vaguely remember a movie from when I was a kid, probably made in the late 70s or 80s. It might be Canadian or Australian?

There were two kids who went through a magic tree in a forest and wound up in another world. There's a guy who serves as their guide, introducing them to a woman (or possibly mermaid?) who is the living embodiment of the rainbow. I remember that she lived in a misty spot, possibly by a pool? I think there was a bad guy trying to cut down or blow up the tree in order to get the pot of gold at the end of the rainbow. Messing with the tree almost kills the rainbow-woman, but the kids manage to save her.

It's not Rainbow, and it's not The Great Land of Small, I've checked. Any thoughts?

It probably aired on Disney or Nickelodeon. Slim chance it was on PBS or HBO.

Thanks for your help.