We will be booking her in in her own name, so they will get their first hint then that it will be a little girl on the day. And a little cheery persistence tends to go a long way. I will make sure I have it smoothed out before she arrives for her appointment, though - just so she doesn't think there is anything wrong with it. She's only 4 (turning 5 on that trip), and doesn't understand yet that others have gendered expectations. I remember the first Minnie Mouse dress I bought for her, she cried - because she liked Mickey, not Minnie. I thought she liked all the MMCH friends equally, but it turns out, no. I looked everywhere for a Mickey Mouse dress for her, and couldn't find anything. Australia is getting better with stocking Mickey Mouse merchandise, but there are only ever a few things in the store. I bought both girls some tops for our trip last week - and had to go to the boys department to get a Mickey top (they had princesses and Minnie for girls). I got the princess ones, too. I am making them Lightning McQueen dresses.
I don't know how long it will be before she starts getting pushback from classmates and peers, but for the moment, I am going to enjoy the fact that my house is as littered with Lightning McQueen and Mickey as it is princesses and Minnie.