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).
I tried an ATV tour with Kini Kini Transfer and Tours last June and I had no problems with the company. Indeed, they picked me up at my hotel on time, I had no problems with the ATV/helmet that I was given, the tour guide was great (he did not go too fast/drive dangerously and he was very knowledgeable) and the tour itself was not rushed. I really did not have a negative experience while on the tour or with the company.
The cost of the tour was $65 for a half day (i.e. from 9:00 AM to 1:00 PM) for a single rider. The sites I visited included the California Lighthouse, Wish Garden, Alto Vista Chapel, Bushibara Ruins, Natural Bridge, Natural Pool (I stayed about a half hour at the pool itself - enough time to go for a quick swim) and Ayo Rock Formations.
BTW - If you plan on visiting the Natural Pool with a tour company, I would recommend going by ATV as opposed to by jeep as I found it to be a far more interesting/exhilarating tour. That being said, it is a little rough on the way to the Natural Pool/on the other side of the island. As a result, if you are not comfortable riding an ATV, I would stay away from any of the ATV tours on the island.
My Review of Kini Kini ATV Tours from a Previous Post
At the advice of Graham Flanagan here on the forum, we took a private ATV tour with Kini Kini last year. By far, one of the best island excursions we have done. We knew specific sights we wanted to visit and Kini Kini gladly obliged. They are a small Mom and Pop operation- an Aruban and his American wife.
"So dd and I sit down to await our pick up from Kini Kini Tours and lo and behold there is their driver recognizable by his "Kini Kini Tours" t-shirt sitting on the bench across from us. We went over and introduced ourselves and he helped us into the van and we were off. A short ride down to the high rise hotels we picked up a couple who were taking a jeep tour.
Our driver Ray took us to Kini Kini's office. It was a house and we were instructed to walk through the house to the back and there was the office! This office is actually in the home of the owners Eloy and his wife Christine. We indeed felt as if we were being welcomed into their home.
Our credit card information was processed and we were then taken to the ATV's for orientation and safety as well as fitting for our helmets. Eloy and Ray made us feel very comfortable and gave us a good run through before our butts even hit the seats. If the ATV's were not new, they were "like" new. During our stop at the Natural Bridge, a rider on a rented ATV from another company admired our ATV's saying, "We got the good ones."
I rode single and our 12 yo dd rode on the back of the ATV with dh. Following our guide Ray, we were off to visit the sites. I have to say dh and I had never rode ATV's previously and were slightly nervous given the prospect of riding some local roads prior to heading "off road" once past the tip of the island and the California Lighthouse. Our worries were for nothing. By the time we left the lighthouse we were of course off road and I know I personally was feeling very comfortable driving. Ray allowed us to drive at a speed comfortable to us, leading accordingly and offering up insight and history at each stop.
Our sites visited included: California Lighthouse, Wish Garden, Alto Vista Chapel, "Baby" Natural Bridge, Bushiribana Ruins, the Natural Bridge and Natural Pool. We have been to all the above sites previously and again, took the tour for access to the Natural Pool. We did give up the stop at the Ayo Rock Formations in order to be able to spend additional time at the Natural Pool. Our guide Ray was very happy to oblige. Dd was happy to be able to take a swim, cool off and rinse off some of the dirt.
On the back of his ATV, Ray carried a cooler with water as well as strapped our backpack carrying digital camera, video camera and towel and was happy to allow us as much time as necessary at each stop for photos and video.
I do have to say after visiting the Natural Pool twice previously by 4 x 4, getting there by ATV, was by far the easiest and quickest way in which to get there once we were "off road". It was still a little "hairy", as it was in a 4 x 4, but by following our guide Ray, we knew exactly where on the path to follow. I might add I was grateful the brakes worked! Never once did I fear for my life though. Imo it is more dangerous swimming in the Natural Pool than getting there via ATV. I cannot recommend this tour highly enough, especially as a way to see the Natural Pool. This was one of the best excursions we have ever taken on the island in the 13 years that we have been visiting.
Was there a downside? Dirty as anything. Riding behind our guide Ray, and then my dh, I literally know now where the expression, "eat my dust" comes from. Remembering how green the island was at Christmas, it was immediately noticeable that it was now so totally opposite. Brown as brown as can be. No significant rain since February, by June the island had become very very dry and DUSTY. As a side note, as I will mention later, the island did see significant rain June 24 and 25. In 17 visits I have never seen the island as dry as it was. If you prepare beforehand and wear junky clothes (think throw away) and shoes as we did, this will not be an issue. My t-shirt from that day just went into the trash; no amount of bleach was ever going to get out all of that dust.
Dd complained about everything under the sun during the ride, but this was her chance to see and swim in the Natural Pool. If you asked her now I think she would say she liked it but will tell you that she did not like the dirt and the dust. For dh and myself it was a unique experience and an opportunity to see Aruba from a different perspective.
Advice- wear "disposable" clothes & shoes. Ours looked similar to Andrea's after horseback riding (same principle )