What does grover tell percy about the big three why are their children in greater danger than the half bloods?

He tells Percy the story about the tree and a girl named Thalia. The story was that Thalia, Annabeth and other half bloods where exploring outside of the half blood camp. But then monsters came out of nowhere and started fighting them. They tried to fight the monsters but they weren’t strong enough. So Thalia protected them and risked her own life to save them. But thalia died and Zeus her father grew a tree in her honor, and the tree provided a barrier for the camp to keep them safe. :-)