They will. The thing about children is that they are wildly uncontrollable and curious. They will show each other their genitals at some point if left to their own devices. And then a few years pass and they will be full or hormones doing it all over again.

At no point does an adult even need to be part of that process. Its one we are literally instinctively wired to do, to the point that even trying to help will fuck it up.