Dinosaur Cove 7 Rescuing the Plated Lizard:

Book Description
When Jamie and Tom emerge in their secret dinosaur world, they realize that something’s changed. The landscape is familiar but also very different from the last time they were here. And when they spot a herd of Stegosauruses, they realize that they’ve switched from the Cretaceous to the Jurassic! It’s the same world but a completely different set of dinosaurs – awesome! A stegosaurus egg is washed away in a rain storm and it’s up to the boys, and their dino-friend Wanna, to rescue it and return it to its mother.