I used to do bridal shows and the little girls there would usually have some sort of updo. Either some up and the back left hanging, all up w/ curls hanging or a bun. We always had someone there doing hair though and honestly the styles were pretty intricate. You could also go for simplicity and make a high braided bun (or get a braided bun cover). Good luck - I'm sure that whatever you end up doing she will look beautiful. Make sure to take lots of pictures!