This is a case which is legally complex. The result here combines Meta's work with the child's work. A lot of the legal test might hinge on things such as the extent to which the gaits (jumping, dancing, etc.) are creative works created by Meta versus automated transformations. I suspect a court decision would hinge on the judge, and perhaps what the judge ate that morning.
But, for the sake of argument, let's assume you're right, and remixing outputs is 100% legal.
Families won't know that. For families to know that, they'd need a legal analysis from a lawyer. Families won't pay for that so their kid can make a Youtube video.
If you're making the world's ultimate video skit, do you want to make it without knowing what you're allowed to do with it?
The problem with legal complexity is simply that people don't do things.
A nice license, being explicit about what you can and can't do resolves that.
License text is also a social contract. A compiler maker might not have copyright over the output, but if they don't want me using their tool for something, I might respect that out of politeness. Or I might choose to pick a fight if I think they're being unreasonable and it will help others. It depends on the context. Signalling intentions is helpful as well.