I am looking for a Sci-Fi novel for young adults about children who discover what is described as a talking talisman that takes them back in time to find its origin. They end up in the prehistoric era where Earth has been invaded by aliens and humans enslaved. It would seem the talisman was brought to Earth by the aliens. The novel is very humourous but serious and I can't seem to recall the title since the book was missing the first few pages and cover when I read it over 20 years ago. I would appreciate it if someone had an idea what book this is. Thanks.