Under the Mountain. This is a book by Maurice Gee, written in 1979. It was made into a TV series in 1981. NZOnScreen has episode 4 available for viewing. It was made into a movie in 2009.
Rachel and Theo Matheson are twins who meet an old man, Mr Jones, while they are on holiday in Auckland, a city built on volcanoes. Mr Jones is an alien who needs red-headed twins to fulfill his mission to stop slug-like aliens (a different race) taking over Earth. He has tried before and failed when the slugs killed one of the boys helping him - this is a dangerous task and Theo and Rachel are his last chance as he is near dying himself.
The slugs also take on human shape and are known as a rather creepy family called the Wilberforces. They can also change shape as needed.
Mr Jones gives Theo and Rachel a stone each (these are shown as egg-sized and glowing red and blue, I can't remember what the book says). The stones have to be thrown over Rangitoto Island (an extinct volcano in Auckland Harbour) to bind the slugs.
Slight spoiler here:
In the original book, the twins' cousin Ricky is killed by the Wilberforces, while he is taking the children to Rangitoto in a jetboat. Maurice Gee has said so many people (including his wife) told him that he shouldn't have killed Ricky, that he would have written it differently now. Ricky lives in the movie. [Sorry, I can't remember if he lives or dies in the TV series. I suspect he dies.]
If you can, try to find the TV series. I think it was better than the movie, though the movie was also good.