Remember in this context I am talking about an enlightened anti-contact view, not the current societal norms.

"Harm" is one step removed from "risk of harm" and it is different from "simple consent" and "children's expressed consent that adults can rely on". It's a societal judgment about what when adults can rely on children's expressed consent and when they can't.

So yes, on this view if a 9-year-old plays sexually with a consenting 9-year-old, even if one of them later regrets it, parents won't try to punish the other one. But if an 18-year-old plays with a 9-year-old, the 18-year-old is in for some punishment. It will be likely much worse if the 9-year-old didn't consent.

Please to show me a case where a judge dismissed a claim on the grounds that the children's mutual consent nullified the parent's preference.

I think in today's society, in meting out whatever punishment might be warranted, a judge looking at sexual activity between two 11-year-olds would take a different view if one of them was doing something to a struggling partner as opposed to mutually desired activity.

The case wouldn't come near trial in the enlightened anti-contact view.

Father to prosecutor: "I want 5-year-old Johnny in prison because he touched my 5-year-old Jenny between the legs". Doesn't fly.

Father to prosecutor: "Then I want his parents in prison because they didn't supervise him enough". Doesn't fly.

Father to Johnny's (sex-positive) parents: "I want Johnny punished because he touched Jenny."

Johnny's parents: "But it looks like Jenny didn't mind. If Johnny had done it when Jenny didn't want him to, we'd punish Johnny, but not if Jenny didn't mind."

On the enlightened view, there are severe limits on how much you can punish a child anyway. If a 6-year-old kills someone (with a rock, say), the punishment should still be quite limited because it's just a kid. Part of why age difference matters is that people over 18 are more responsible for any bad thing they do.