Not allow any upload to be viewable by children until it's been reviewed by a human for basic content guidelines. It would be as simple as not allowing any upload that uses licensed characters not directly owned by the uploader, and not allowing any video with disturbing or age-inappropriate content. This would cut the uploads down by a factor of a hundred or a thousand. Okay, fine. Reimburse them for views at 10x or 100x the rate of general Youtube content.
Congratulations. You're now incentivizing professional studios to create children's content and disincentivizing the off-shore CGI farms.
This shit gets so easy once you ditch the libertarian "we're dissolving the boundaries between creators and consumers" mantra Silicon Valley clings to like a goddamn bible. Guess what, choads - I want a fuckin' filter. I want some curation. Know why I let my kid pick whatever she wants off PBS Kids? It's f'n curated. The fact of the matter is, when you refine your platform to make it reward abso-goddamn-lutely anything that gets uploaded, you'll get shit like Elsa burying spiderman alive.