There is no evidence to suggest that such a horrible event involving Beto O'Rourke running over children has occurred. In the world of politics, false stories are sometimes spread to smear a candidate's reputation. We should be cautious about believing such unsubstantiated claims.
Most likely it's a fabricated story with no real origin. It could be started by someone with a political agenda against Beto O'Rourke to spread false information and damage his public image.
I haven't seen any reliable reports about Beto O'Rourke hitting children with a car. It's important to base our beliefs on facts, and until there is solid proof, we should consider this story as untrue.