Except, we all know from watching the news on a regular basis, that sex with children IS physically possible. The only reason why it isn't more common, is because kids that age have no real desire to engage in it.
By "child" in this case, I'm assuming you mean someone who has not yet entered puberty. And in that context I'm generally agreed.
If pressed (say by a brother, father or cousin) they may consent, but they'll rarely initiate it, and most times they'll avoid it, even when being forced to consent.
Back to general agreement, assuming 2 things:
1) The child in question is sexually inexperienced.
2) The child in question, if sexually experienced, hasn't had a "pleasurable experience" while engaged in the act. (and considering many, if not most people, are "bad at sex" that isn't hard to imagine)
For most children, their main interest in things will be "is it fun?" and "does it make me feel good?" (And making them feel good is likely to make it "fun" for them. As that is an all too common crtieria given by children for why anything might be "fun")
For a child that simply walks in on someone having sex, they're likely to simply declare the typical "Sex is gross" response and go elsewhere, as you mention.
But in the event someone is around without scruples, and they work the child past the "it's gross" stage, the situation is likely to change. I'm not saying that is a good thing, it just happens to be a thing)(with a long list of reasons why NOT to do so). So it still runs back to "Someone deliberately introduced a child to sex" whether that someone is a 30-something pedophile, or a 13 year old introducing an 11 year old who then initiates a 9 year old in turn.
It just happens that the odds are very good that the 13 YO will be terrible at sex(like most young people), so the 11YO won't enjoy the experience, and that'll be the end of it. Or if the 13 YO is somehow good at it, the 11YO won't be as skilled, so the 9YO won't enjoy it, and it ends there.