You'll be able to find plenty of resources through your local office of Social Services and you can call the toll-free Child Abuse hotline and ask the counselor on duty what your options are.
Whether your daughter mentions the incident again really makes no difference as to the damage that's been inflicted - I'm sure you've read about or known someone who was molested as a child and never spoke out about it until they were adults. Nevertheless, the impact on their psyches is very traumatic. If she's allowed to keep discussing it with a trained professional, she will learn many tools to navigate through her life. Your son will benefit from the same counseling, without a doubt.
I completely understand how confused you are, right now. That anyone (any one) would imagine that children performing oral sex on one another as "...no big deal..." is a screaming, flapping red flag - someone's EXCUSING this behavior.