Something to understand about trademarks is that since they are an indicator of the source of the goods, there must be quality control provisions in place for any licensees. Its not clear to me if you are using a Disney-licensed costume, or if you made or purchased a princess costume which is basically a knock-off. If you are able to get a licensing agreement, Disney will certainly require you to wear an authorized costume. In order to make sure that you're up to the "Disney standard," I wouldn't be shocked if they made you attend their own training class before granting you a license.
As far as your actual inquiry goes, you'll have to contact Disney's licensing division to get directed to the correct person/place. The following website may be of some assistance, at least in giving you a starting point.
This is for manufacturers who want to propose licenses for Disney products, but it may at least give you an address to get started with. You could also locate the phone number for their corporate headquarters and try to get connected to the licensing department. There has to be someone there who facilitates this type of license grant.
Hopefully this helps and good luck.
Disclaimer: In no way is this information considered a legal advice, but is only a statement of the law. This information does not form a client-attorney relationship.
Disney is very protective of what they view as their "intellectual property," so even if you not infringing their copyright or trademark (merely an assumption at this point), they may still turn their attention to you.
Bottom line: if you intend to make money off of this venture, you need to talk with an IP attorney sooner rather than later. With all of the facts, they can advise 1) if you are acting in a legal manner, and 2) regardless of if you are acting legally, is Disney likely to take action against you.
- Neil Wehneman
P.S. The standard disclaimers apply. These are general statements of law, and not legal advice, because I do not have all of the relevant facts. I am licensed to practice law in Indiana, and only practice outside Indiana through association with local co-counsel.
If you have other questions or if you would like to discuss your situation in more detail, feel free to contact me.
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Your question seems to assume that Disney is obligated to give you a license. Not so. Disney could decide that it does not want you to appear as this character at birthday parties. Do not assume that this will be an easy or inexpensive matter to resolve. My best advice to you is that if you want to continue to profit from use of this character, then you need to retain IP counsel. You definitely should not try to deal with this on your own.