I can remember clearly the first time my parents tried to get me and my brother on Big Thunder Mountain. I was crying and my brother had a death grip on the chain at the entrance!! But after the first ride, well, we jumped off and got right back in line to go on it again! My parents knew us and they knew that we could handle a bit of encouragement. Only you know your kids and how far you can push them so use your best judgment.
Live and learn, you have to build on the day, not start with the fastest coaster in the park right out of the gate!