If search is working, check out the many, many threads on the "whether she should wear her princess dress" question.
The overall concensus is if she really wants to, or if she'll feel left out when seeing other little girls in princess dresses (some but not all of the guests) then let her be royalty to her hearts content....provided you bring a change of clothes along.
Be aware that there are "princess carts" and shops all around which will try to sell you high-priced princess ware to outfit the little girl who arrives in shorts. Much better to bring your own than have a big fight just when you hoped for a magic moment.
Some families work a compromise, like a princess t-shirt or a tiara. Best advice seems to let the Little Princess figure out what her heart desires.
It's often experienced that the characters are more lavishly interactive when responding to a little human princess who dresses like them.