Worst pain ever...

Okay a few days ago we decided to let's the kids have slumber party. We thought it would be fun, great time. So the next day the kids were out swimming at the pool and were riding bikes. I had just left the house to go to work. My bf calls me telling me to get home now! He's screaming at me to come home. He tells me that one of kids was hit by a car in the driveway that he couldn't save him. He die. My heart literally fell out my chest. I rush home.. To the worst scene imaginable. What he hadnt told me on the phone was that my bf father had been the one whom had killed the boy. Or that my bf had climb under the truck and held the boy til he died. I've been taking care of he's family and he's kids the last few days. My bf acts like he's okay that's he's making peace with it. But in my heart I know he's in pain. I've so much guilt bc I arranged the slumber party. If I would've been there I could have saved him (I'm a nurse). Everyone is telling me it's not my fault... I just wanna help my bf and he's family. But no one can imagine how much guilt or blame I'm putting on myself. I'm so lost! How can I be strong for everyone else when I'm losing it inside?