Help: Halloween costume

I bought baby moana costume for my daughter. The daycare is very adamant that I put her clothes first and the costume on top. It doesn't serve any purpose. The teacher says she doesn't have to dress 16 kids after the parade that way even after a personal note. What do you think I should do? Any suggestions?

Edit: it is a big deal because Moana has a little top and a little skirt (google if you don't know). The costume won't look the same with layers at all. I was asking for suggestions like sheer tops or if I can stay at daycare if changing her is an issue.