Just a note re: kids who are exactly 40 inches tall. (Not shorter; I am not advocating ideas like elevator shoes). Beware of "afternoon shrinkage" on their height, as kids tend to droop a bit as they get tired. Most rides with a height limit measure kids twice, at the entry and just before boarding. Sometimes the bars are a bit over 40", and it is totally up to the CM if they get by or not. The CM's perception of the child's readiness can make all the difference as to whether your child gets refused at the second measure. For one thing, do not carry a child that size in the ride line. IME, a child who is just at 40" is more likely to be turned away if he/she was being carried in the line. Also, if at all possible, practice with a simulated L-stick, so that your child knows that the object is to HIT the bar, not duck under it.
That said, only you will be able to judge what will be too intense for your child. I always like to start at the Studios with Star Tours; it's less intense and tends to be funny in spots, which helps kids who are not sure what to expect. Barnstormer is a good starter coaster, as is BTMRR; be careful of SpcMtn if your child is afraid of the dark (though you CAN keep a hand on a child on SpcMtn if you sit behind him and put your hands on his shoulders.)