We were at BBB just the other day with granddaughter who will be 8 next month, she wanted to be her own princess so she did not wear a Disney princess dress but a gown and heels and was asked many many times for her autograph. 1) she was not the youngest, there where several her age getting done that day, I would say average age was 5 to 6. 2) there were several about age 3 and none sat for more than 30 minutes, more like 15 to 20, my granddaughter took the longest of all of them, her hair is very thick and is down past her butt, so yes it took her the longest, we spent 30 minutes, that included dressing, paying, tipping, hair nails and makeup. 3) you can choice not to use makeup several of the younger parents did say no to the makeup. 4) they do not pull on the hair rough like some are suggesting, it was gentle and kind, the gel actually makes the hair seem pulled tight, if there is not enough hair the FGIT does a great job making it look just like the older girls even if it is really not flipped under.
Anyway, I would call tell the truth and see, make appt if they say okay, or don't if they say no. We go every year to dlr and every three years to wdw, for us we waited for BBB and CRT till this trip at age 8 next month. First she has beautiful hair and never needed it as a little one, her hair has always been long and curly and looked like a princess even when done by us. Second, price. Third, we wanted her to remember her day, this year she is old enough to remember the day, old enough to say which princess she wanted to be, rather than love them all and I love she picked princess Aaliyah, herself, and four she is old enough for CRT and can remember it.
We had a great time, if you are not going to the park for years and years have it done, other wise wait till she is a little older.