The Cave of Dreaming - The Altar of Shulaani, #2

Mike, Alinea and their alien pet, Beeper, are expected to stay out of trouble during their holiday on an archaeological dig in Central Australia. While in the cave of the dreaming, they inadvertently trigger a time gate that sends them to another universe else entirely! To get back to the dig site, they'll have to escape an intergalactic lost property office until claimed, help their new friends, and avoid being eaten by the deadly Kloog raiding the protected planets--again.
Somehow Mike doesn't think his father will accept "It's not our fault!" if he finds out they haven't been where they were supposed to be the whole time...