You don’t need all of the answers when you report. Avoid digging for details or having different adults question the child’s story. This can re-traumatize the child and damage the investigation and/or prosecution.
In making a report you don’t need proof. You need reasonable suspicion, which means that you have witnessed abuse, become aware of or believe a child is unsafe.
It also could mean you have observed a pattern of boundary violations and you have intervened, but the person continues to violate boundaries. “Grooming” is and example where an abuse continuously builds and inappropriate relationship. To learn more about grooming and other 'red flag' behaviors check out this resource from our friends at Darkness to Light.