I remember watching a TV series in the 80s about a boy and a girl who are on vacation on an island - may have been New Zealand, although I think they had Australian accents - and they run into a creepy family who are actually sort of slug-like aliens who can shapeshift to look human.
I think the series was based on a book, but am not sure.
The aliens have underground tunnels all around the island and are trying to steal either all the life force or all the mineral resources from the island or maybe from Earth in general, not sure.
There's another alien who is apparently on Earth to try to stop them. He recruits the kids and teaches them to use some sort of psychic powers they have to communicate and use some kind of psychic energy weapon to destroy the aliens or the gateway they use. They also had to say something like" I give you the gift of oblivion" I think?
A couple of scenes or plot points I remember:
The aliens owning / living in a house accross a lake from where the kids are staying that they can see from the house they are in
Scenes with the kids having to concentrate on some rocks to make them glow red or blue and the boy initially being sceptical about the whole thing.
At some point the boy lets something slip that makes the aliens suspicious and almost gets them all killed.
Scenes with one or both of the kids using/sliding down the alien tunnels that looked rather organic and kinda creepy and ending up in an underground lair.
At one point they find some sort of hologram or model of the galaxy floating in a room in the aliens' house or underground lair