The “ideal” way this would work is that your daughter’s father would seek out therapy for his son immediately, and that you could both contact DCFS together to let them know that it has been discovered that there has been inappropriate sexual activity between the two children but that therapy is being sought and a safety plan has been put into place so that neither child is at risk of any future sexual harm.
Your daughter’s disclosure would be appropriate to report to your local child protective services (CPS). Reporting These are indeed complicated situations and there is not easy answer regarding your next steps. It's great that your daughter told you what is going on but now the adults need to step in to help all the children involved. I’m so sorry to hear about your daughter’s disclosure and I’d like to start by suggesting that you read our Online Help Center information, When a child tells about abuse.