(Personally, I think that, if the girl is too little, she might use the familiar form even though Superman is an adult she doesn't know well... but I might be projecting from Greek. Anyways, polite should be okay.)
Chiming in here a little late. I'd say she'd definitely use the polite form. Superman is basically a god to people who don't know him, and she would be in awe. She'd have been using the usted form from infancy anyway, because a lot of Mexican children, especially rural ones, address their parents with "usted", not to mention grandparents and other people.
As an aside, a lot of writers don't try to translate dialogue into another language. It's actually easier for the reader if you give one sentence in the target language, then say something like "Superman continued in Spanish..." and write it in English. Since not all your readers will speak Spanish, and putting translations in a story is somewhat awkward, it's okay to just do it in English. We will all know that the two are speaking Spanish with just a few indicators in the text. Just something to consider.
