Videos
Does the name Dinosaur King ring a bell? It has a group of kids finding a series of tablets in the woods, and it also involves time travel using what are referred to as Element Stones.
(It looks like someone beat me to the punch with this answer while I was writing it! Sorry about that, I'll leave mine here for extra information!) There are a lot of dinosaur cartoons, but from your description you may be referring to the cartoon Dinosaur King which aired between 2007 to 2008, and focused on a trio of kids who each had their own pet dinosaur and called themselves the "D-Team." Each dinosaur had its own elemental attack, and could be summoned from and stored within a card using a special stone. The kids frequently faced off against a group of stranded time-traveling scientists. The D-Team try to protect their new dino friends from the scientists, and so it becomes a battle for who can collect the dinosaur cards first.
This also has some episodes on Youtube, including the first: