What you describe is typical childhood behavior of "playing doctor" and is perfectly innocent. In most cases, a child that young doesn't even know what sex is! Sure, they find some things feel good, but that is not sexual.
As you say, it's not normal for such a thing to result in trauma later. What I suspect must have happened is that one the parents involved found out and freaked out and went ballistic, not only on their own daughter, but the playmate as well. So, it's not the event that upset the girl but the adult's reaction to it.