I don't see any safety concern for a healthy childs' neck on RnR. It's not like RnR is a paint shaker where your head is grabbed and shaken around violently. It's actually quite a smooth coaster. I'd have far less concern for their neck on RnR than I would on BTMRR or Dinosaur, which really do shake you around...a lot. Second, as ravenclawtrekkie mentioned above, there's a height requirement for a reason. I'd bet that this was studied closely by engineers and consulted with physicians, etc...as the ride was being built and designed.
Sure, if a child has a pre-existing medical issue, that's one thing. But an otherwise healthy child, I don't see it. Kids are far more resilient and flexible than adults.