We've only done it once, but definitely plan on going again. DD was I think 4 years old when we went, and she did fine. She was a little disappointed that her Pluto didn't turn out as well as she would've liked, but it wasn't a big deal and we all had fun.
I don't know if this is standard policy, but our instructor had us vote on which character to draw. (I think he gave us a choice between Pluto (the winner), Daisy, and one other character.)