I remember a book I read when I was about 12, and it was targeted to juveniles. In it, there were two (maybe three) young siblings (brother and sister I think) lost in time trying to get home, I distinctly remember them running through a forest in pre-dinosaur age earth (the Cure's "Lost in a Forest" always makes me think of this book :). I don't remember much more about it. Can anyone id it?
A few stories that could possibly meet the description (in descending order of likelihood):
Joan Phipson's The Way Home (1973). The story features 3 siblings (Prue, Peter and Richard) who survive a car accident in the Australian outback in which the driver is killed. Trying to find their way home they set off following a river but as they walk they find themselves passing through different times, including a prehistoric setting with volcanoes and such like.
Dinosaurs Before Dark by Mary Pope Osborne (1992) Brother and sister Jack and Annie travel 65 million years into the past (not really pre-dinosaur, so much as just full-on dinosaur) through one of the books in their magical tree house.
If you were 12 somewhat more recently - it could be:
Timespinners by Luli Grey (2003). The mother of two twins (Allie and Thaddeus) is in a coma, the twins end up going to the museum of natural history and falling into the past through one of the dioramas. Eventually they end up in Paleolithic France (which I realize is significantly later than `pre-dinosaur age earth')