Please. Do not leave them alone together. Even if nothing more happened it needs to be treated like a drug. They won't do them in front of you. The nine year old my try a little something more next time. Be present. Don't announce a visit that your coming around the corner. The 9 year old may not know what he is doing but just curious. None the less, if the 3 year old never remembers anything, the 9 year old will harbor a lot of guilt. It happened in my family and now 10 years later he feels awful. The 5 year old at the time remembers everything and even though she 'understands' his guilt, it's still not comfortable for either of them. We adults didn't know anything until it went too far and the 5 year old actually approached us about it knowing how wrong it was after he threatened to tell the devil on her if she said anything. It could be curiosity or in our case just cruelty. Hope this helps.