When Dinosaurs Ruled The Basement — William L. Deandrea

TRAPPED...IN THE LAND OF THE DINOSAURS
Twelve-year-old Jon, his little brother Michael, and his best friend Gemma have made a discovery so amazing no grown-up would ever believe it -- not without proof: a baby dinosaur is loose in the basement!

In search of the proof they need, the kids step into a time warp...and can't get out. Now they're running for their lives to escape an Allosaurus and getting deeper and deeper into an overgrown world filled with all kinds of dinosaurs, large and small. But even the largest dinosaur is nothing, compared to the terrifying monster they encounter next!