Well if Disney made the same amount of money as the high school kid, and relied on your song to the same degree, then presumably they would pay the same.

As for what percentage of the derived-work revenue needs to be paid to you, just let the deriver pick something that they think is fair. It'll be part of the metadata on that work so everybody can see how much of what's being paid is being passed along.

Then we create a culture of amplifying works that are fair to their dependencies. As for those that are doing and bare minimum, we name and shame (and sabotage?).