You wouldn't sound like a busybody. Don't e-mail, don't make a phone call, but go directly to the school and request a sit down with the principal. They may be aware of the situation but they may not be. The fact that the boys told you what's going on and in such detail means that it's affecting them deeply and they're afraid, and it's a cry for help. Be the adult that listens and gets help for them.
This way the school can get a counselor and school social worker to talk to the boys to start an investigation and they are mandated reporters. They are bound by law to protect these boys, so if something hinky is going on the police and CPS will be brought in and the school will be the ones to do it. You will be left out of it and the family ie. the father won't ever hear or see your name or your children's.
We had a similar situation last year, except a 3rd grader told my then-5th grader that her mother was beating her and emotionally abusing her. Talking to the principal was the best thing I ever did. Neighbors apparently knew about this ongoing issue and never called the police nor CPS. No one ever got involved because it "wasn't their business." But those kids in that house needed help. The school had an idea because the kids were always in trouble for bullying and other issues, but had no proof until I went in with what the girl told my daughter. The school investigated with the counselors and social workers, interviewed the children (the girl and her brothers, plus my daughter) and being mandated reporters found that there was indeed reason to call in CPS/DCYF.
I don't regret it. The principal will take down what you say and handle the rest.