Sometime in the late 70's or early 80's I read a sci-fi novel, the title and author of which have completely escaped me. I have a vague recollection of the plot, which I will describe here but please note that:
1) I may not remember it 100% accurately
2) The description contains a spoiler of the novel's ending.
The story concerns two children who, while exploring, discover a cave guarded by some sort of supernatural creature. Discovering the creature to be only an apparition, they venture inside and discover a spacecraft. Inside the spacecraft are a sleeping man and woman. The woman awakes and tells the children that she has flown from somewhere far away, and that her spacecraft is powered by a strange substance called "Plaax", which in turn was discovered when aliens visited her home planet. She wishes to revive her husband/partner whose name is Gifon, as he is in some sort of stasis/hibernation.
Ultimately, the story concludes with Gifon being awoken and the couple depart in the spaceship, leaving the children behind on Earth. The novel concludes with the children concluding that the couple did not come from another planet, but from Earth's future ("they came from the future!" being (a paraphrasing of) the last line of the novel).
Does anybody recognize this novel, and know what its title and author might be?