Magic Tree House 7: Mammoth to the Rescue by Mary Pope Osborne
Eight-year-old Jack and his little sister, Annie, are playing in the woods when they find a mysterious tree house full of books. But these are no ordinary books . . . And this is no ordinary tree house . . . Jack and Annie travel back in time, wearing only their swimsuits! When the tree house lands in the freezing Ice Age, they have to hide from hunters and escape from a great cave bear. But when Jack and Annie are chased by a fierce sabretooth tiger and fall into a hunting trap, they're really stuck. Will anybody come to their rescue?