On our last trip in October, we had planned on skipping the roller coaster rides for our 3 year old DD, including the Barnstormer, even though she was more than tall enough at 40-inches.
However, during a side trip to Universal IOA, she begged to ride the Flying Unicorn (she loves unicorns). Being a roller coaster fiend, I was excited, but the DW was less than thrilled. After a minor debate, won by the pleas of the DD, we climbed aboard the front car and away we went. We ended up riding it eleven times in a row, much to the amusement of Universal staff.
After that, we let her ride anything she wanted as long as she met the physical requirements, including the Barnstormer and Thunder Mountain Railroad. We also found out she loves to ride the Tea Cups with me and wants me to spin it as fast as I can. My wife refuses to ride with us now.