Since she's under the age of 12, convention policy states that you have to be with her at all times regardless of what she's competing in. Plus it would be lovely to have you backstage anyway! All contestants stay in the Green Room when not onstage, because you never know whether or not you're going to be called onstage to accept something at the end!
The rules state that for her costume to count as a 'youth' level craftsmanship, she would have to have made at least 50% of the costume herself. If she made less than half of it, depending on the skill level of yourself and her grandmother you could have your pick of Novice, Intermediate, or possibly even Journeyman.
You will have to be present at craftsmanship judging, both to answer questions about her costume and how it was made and to accompany her because she's a minor.
Hope that answered your questions!
