You did the right thing by telling the parents. They seemed concerned and ready to take charge of the situation. If you can, maybe be available in case either of the kids or their families needs to talk or needs more information, but let them approach you. And if you don't want to be involved, just tell them that you already told them all you knew and don't have any more to say about it.

If one or both kids has been sexually abused, one of the best things that could happen for them is to have an adult around who can just listen to them and be a friend and validate whatever feelings they might have that need validation. It is important for all kids to have someone they can trust and communicate honestly with without fear of being judged or misunderstood. I don't know if that is a role you are interested in or if that is a possibility. You don't owe anyone anything more.

You didn't witness something that you are required to report officially. Because the kids are the same age, one probably would not be considered a victim. They were experimenting. And it's sad, but they are getting younger and younger and knowing things they are not enough to know. What they see on TV and the movies probably has a lot to do with it. If I were you, though, I would talk to my T about it. Especially since it seems to be bringing up something for you.