Mary Pope Osborne 
Magic Tree House 1: Valley of the Dinosaurs 

Eight-year-old Jack and his little sister, Annie, are playing in the woods during their summer holiday, 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 get more than they had bargined for when Jack opens a book about dinosaurs and wishes he could see them for real. They end up in prehistoric times with Pteranodons, Triceratops and a huge Tyrannosaurus Rex! How will they get home again? The race is on . . . !