You need to book it when you arrive. They do not take advance bookings. When we arrived I headed straight there and made the appointment for a few days later. You pay when you are making the appointment, so be sure to bring your purse!!
Our daughter loved it! She was walking on air for the entire day.
My girls have done it before too - and loved it! Have to say, with the full package of dress etc, it isn't cheap, but if it's in your budget then go for it.
If you do a search on this forum for the Princess makeover you should find lots of posts from people whose children have done it (inc price lists) and lots of nice photos too, which will give you some ideas . I think it used to be called 'Princess for a day' makeover, not sure if it is still is.
We were at DLP from Friday to Monday just gone, I went to book this for DD and was informed it's closed until October 18 (2 days time). They did have face painting in the park though, just near Auberge de Cendrillon so we went with that instead.