I think if the child is within about an inch of the height restriction, the CM might allow him/her. When I was there last week, I saw a little girl on her tip toes within about an inch short and the CM waived her through with some begging from the child.
If your child can make it throgh the height restriction, he should do OK. The ride is not scary. I have a 4 yr. old not so brave girl and she did fine. There is a slight jerk during the take-off but pretty stedy from them on.
Do bring a light jacket for the kiddos. There is a constant wind during the ride and if you've been out in the heat all day, it can get cold during the ride.