It is likely that Simon was quick to volunteer to go tell piggy about the littluns in the ark because he was desperate to get away. It is clear he knew that the only beast was the boys and it is known that he has made previous trips to the forest before and has a secret place. He would much rather face the forest alone that continue to spend time with the boys, or beasts. It is likely that he didn't think Piggy or the littluns were dangerous due to their lack of participation in violent actions.