I remember watching this movie as a child. I was born in 1990. We had it on VHS. The main plot was 2 kids, a brother and sister, are not at school (sick? Or on vacation?) at a relatives house and going through things in an attic or basement. They find some sort of jewel or necklace and it puts them into a different world. (NOT DARK CRYSTAL). I specifically remember a waterfall in this other world and forests. They have to find something to get back home. There’s “bad guys” trying to find them (perhaps one was a snake? Maybe puppets). I remember at the end they find a cave and there are places all over the room where more jewels would fit and click into place. They start putting them all in and little holograms of other children appear. (Maybe releasing them from the world back home?). Then they return home.