COPPA compliance includes no marketing to children, even with parental consent. So they'd have to operate a completely different platform for children. Plus their business model is based on marketing, so they'd take a huge loss operating a second platform for kids. It is stupid to expect Google to accommodate children.
It's not really a second platform, is it? It's more like adding a single configuration bit to an account that determines whether it is eligible for marketing campaigns (or whatever else it is they can't do with children).
It's even likely that they already have such a thing in place for debugging purposes.
I'm intrigued by this system you suggest that manages to get an entire engine for obtaining 'verifiable parental consent' squished into a single configuration bit, along with an engine to directly notify parents, to allow parents to view all the collected data, and provide deletion from backups. That's one busy bit.
So more accurately: A US Federal Law Made My Daughter Cry