Here's a great little series called Dinosaur Cove - about two boys who discover a secret entrance to a prehistoric world where dinosaurs roam. Each book is an 80 page paperback chapter book, appropriate for children 4 and up.
The author's name is... "Rex Stone." I don't know ....does anyone else smell "Pen Name" when they see a dinosaur series written by someone named Rex Stone? At least it wasn't Rock Fossilman or Terry Dactyl.