I think that there is a big difference between a 12 year old and a 5 year old when it comes to being a passenger on an ATV/motorcycle. Some jurisditictions in the United States recognize this fact and prohibit young children from being passengers on motorcycles. For example, Arkansas prohibits passengers under eight, Hawaii prohibits passengers under seven, Washington and Louisiana prohibit passengers under five (many other jurisdictions are contemplating similar legislation). The laws were passed/are enforced in these states for a reason (i.e. there is extra danger involved when very young children are passengers on the back of motorcyles). Therefore, just because young children may be allowed to ride on the back of ATVs/motorcycles in Aruba does not mean that it is safe to do so. I did not bring my young daughter on the ATV tour that I took for this very reason.
BTW - Aside from the safety issues that have been raised, I do not think that it would be possible for a 5 year old to sit on an ATV for even a 2 hour tour of the island. Indeed, while I found the tour to be a fantastic experience, I would venture to guess that it would be too hot, dusty and uncomfortable for a very young child.