Just because a child is tall enough to go on a ride, doesn't mean it's appropriate for them, IMO. You must judge that for yourself, as a parent, to determine what a child should ride.
That said, IoA is often marketed to an "older" audience. By that, I mean older children and teens. Since the park features many high-tech coasters and other rides, young children (and some adults) are often frightened by them (if they can even go on).
The park does have Seuss Landing, pretty much their equivalent of Fantasyland. It is colorful and there are several rides your child can do there, including Cat in the Hat and Caro-Seuss-El.
In The Lost Continent, the Flying Unicorn ride is new. I have not been on it so can't comment.
In Jurassic Park; Camp Jurassic, Pteranodon Flyers and Triceratops Adventure are probably all fine. The Triceratops is rather big, so just remind your child that he is harmless. The Zookeeper will ease your childs fears too, if there are any.
At Toon Lagoon, the Me Ship, The Olive is fun but be prepared to get wet!
For what's a no go - Well, it depends on your child. All the big coasters are out in the open but require 54" in height to ride. Spiderman has a height requirement of 40" but is a very intense ride with 3D and motion simulator technology combined. You might want an adult to go on first and determine its appropriateness. Your child is just shy of the 42" height requirement for Jurassic Park River Adventure but I'd be wary of recommending this for a 3 1/2 year old. Too intense, IMO.
[This message was edited by Margie J on 01-12-01 at 11:51 PM.]