There are many reputable company's that provide ATV tours of the island. My personal favourite is Kini Kini and they provided me with a fantastic tour of the island (probably the best that I have had in all of my years going to Aruba).
That being said, I would not take a 5 year old on an ATV trip as it would be far too dangerous for them. A five year old would not be able to hold on to someone on some of the rougher parts of the tour e.g. the "path" to the Natural Pool. Indeed, there were a few times that I almost turned my ATV while on the tour (and I have been on an ATV before). Turning an ATV/falling off an ATV at even a slow speed could result in serious injury to the driver and or passenger aboard the ATV. Any type of injury would ruin your vacation.
I would suggest that you plan a jeep trip instead or rent a car/jeep and bring a car seat with you (or rent a car seat from the rental agency) to make sure your little one is safe. If you really want to take an ATV tour, I would suggest that you leave your child in the care of a baby sitter while you go on the tour yourself.
One last point, safety is the main reason why some tour companies do not allow children on ATV tours. I have to question how seriously a tour company takes safety if they allow young children on their ATV tours.
