The mom made the kid tell the elders and the elders forced it out of the perpetrator. Penitence would be if either had come to the elders willingly. The child was a victim and disfellowshiped (lovely, lovely religion that it is!) and the elders didn't think they needed to report a full grown woman having sex with a child.
EVERYONE is supposed to report. That means them. The mom took her sweet time, btw, but at least she did the right thing ultimately.