I agree with Sally. If possible trace an outline of your niece's foot. Judging by my son's Nike and Vans sports shoes, US sizes are about one full size higher than the Australian equivalent e.g. US10 equals an Australian 9. One bit of advice when trying on Ugg boots they should feel very firm as they tend to stretch (or more likely the lining flattens) after a few wears.

I bought them for my daughter when she was living in England from a local sheepskin shop so the one on the Great Western Highway in the Blue Mountains would probably be the best place for you to try. Some of the larger souvenir shops also stock them. I remember seeing them at Darling Harbour and the Rocks.

Enjoy your stay in Australia!

Regards, Josie Figtree, Australia