Atlantis Attacks — David Johnson

Ten thousand years ago:
When Atlantis, a sprawling city on an island in the Atlantic Ocean, suddenly exploded and sank and the Earth was thrown into chaos, several hundred of its technologically advanced survivors were forced to take emergency action and escaped in a fleet of spacecraft to a distant planet forever. But some of the survivors had other ideas. Escaping to Hollow Earth, a network of caverns and tunnels built deep into the Earth and filled with experimental laboratories and research facilities, they put themselves in suspended animation in order to survive the destruction on the surface. And now thousands of years later they have awoken...

Present day:
Another normal school morning for siblings Anne, Lucy and George, or so they think. Going about their usual pre-school routines, they suddenly find themselves confused and terrified...and on a UFO! Who are their strange abductors? And what can they possibly want with them...?