In terms of tznius issues with dressing up, from what I’ve heard it’s not so black and white, and while the Gedolim might say that some (over Bas-Mitzvah) girls should not dress up, they will allow it for others. It depends on the girl’s background and community, etc. There are Bais Yaakov high schools that do allow the girls to dress up (as per the psak of Gedolim and Rabbanim who allowed it for those schools at least).
Lightbrite is at a stage where she needs to take things slowly, so it may not be such a good idea at this point to worry about things that are not clearly assur and may be a matter of minhag hamakom.
That being said, one does have to make sure that the particular costume is tznius and in good taste. I think that LB will have to use her judgment and/or speak to her LOR (probably Rebbetzin in this case).