Does it apply just to the fairy tales or would it apply to any sort of "fan fiction", because there are thousands of YouTube channels of people making videos with Disney dolls and keeping their Disney names. For example,
Ursula Kidnaps Ariel
Frozen Elsa and Anna Go on a Picnic
Frozen Elsa Gets Married
etc. etc.. etc..
Does what apply to just fairy tales or fan fiction? Fair use, or the risk of suffering the potential wrath of Disney?
Actually, both apply.
I apologize because my earlier link to Stanford's "A Fairy Use Tale" was faulty and I have since fixed it. If you were frustrated in your attempts to access it before, it should work now, and it is helpful in explaining what uses of rights-protected material can be all right and what can be risky.
Fan fiction and the use of Disney dolls to create new "adventures" for film characters can either be viewed as a fair use of Disney material, or Disney might see it as infringing on their rights.
It is true that Disney owns the exclusive rights to create "derivatives" of the film Frozen, and fan fiction and creating new tales for Disney characters often crosses the fair use line into derivative territory. Disney could potentially sue anyone they feel infringes on their copyrights or trademark rights.
That said, most copyright holders who believe a published work infringes on their rights will simply file a Digital Millennium Copyright Act (DMCA) notice with the site that hosts the material, and the host site will remove it. No lawsuit, or even a threat of a lawsuit will result.
Disney is a company that is known to police the marketplace for infringers and take legal action against them. The NFL is, likewise, a bit touchy about those who use their rights-protected material without authorization.
But some film companies, record companies, authors and artists are either not as diligent when it comes to guarding their copyrights and trademarks, or they do not find infringement in all that they see.
You can have your fairy tale videos, using Disney characters to act out the roles, reviewed personally by an attorney in your area, to get a better assessment of the legal risks you could face in publishing the videos on YouTube. Or you could create your own characters to illustrate the fairy tales.
Disney is not necessarily a company I would mess with but, whether Disney would notice your videos and have them pulled from YouTube, or challenge your use of their characters with a notice of infringement and legal action, or find your use a fair use and do nothing, is something I can't tell you.