I agree with Arubalisa and would recommend Kini Kini for an ATV tour (In my opinion it is much better than a jeep tour of the island). Kini Kini gives a great ATV tour of the island and the tour they offer is well worth the cost. The half-day is 4 hours ($95 total for 2 people) and the full-day tour is 7 hours ($125 total for 2 people).

The ATVs/helmets that Kini Kini uses are in good working order/safe and the tour leader does not push you beyond your driving ability. That being said, your safety on the tour depends on your ability to control the ATV and drive safely (i.e. the tour is safe if you do not do anything reckless e.g. tricks while on the tour).

As for full-day vs half-day tour, I would go for the full-day tour, however, you may want to ask yourself whether your son is old enough to handle 7 hours on the back of an ATV (although there are stops along the way to see the sites) before booking a full-day tour (I think a full-day tour would be a little tiring for a younger child).