There are some questions about very young kids, roller coasters and brain development. Particularly jerky coasters (like BTMRR or the Dinosaur ride) as opposed to smoother riding coasters (Barnstormer). I don't think any studies have proven there is a problem, but the fact that people who study brain development think their might be is reason enough to give me pause. From what I understand (and I am certainly NOT a neuroscientist - although my FIL is) brain structure changes sometime in the toddler/preschooler range (yep, right around 3 - though obviously there is nothing magical about the third birthday) making them suspect coasters are more dangerous until that change happens. What they suspect is the same sort of damage you get from "shaken baby syndrome" only on a much less dramatic scale.