When buying an ATV for your child, first of all you need to look objectively at your child and make sure he or she is ready for the responsibility of riding an ATV. Some children are mature enough to handle an ATV at an earlier age and others still aren't ready to handle an ATV at the age of 16. It really depends on the individual.
Next, you need to determine fit. Never put a child on an adult sized ATV and never put a child under 16 on an ATV over 90cc. Make sure the ATV has a speed limiter and a kill switch so the speed of the ATV can be limited while your child is learning to control the ATV.
Also, check steering radius and handling. The better the steering radius and the easier to handle there is less of a chance of your child losing control of their ATV. The braking system is another considering. Raceway ATVs nearly all have front drum and rear disc brakes for added safety when stopping.
Additionally, get your young rider some lessons from a seasoned ATVer. Most states have lessons for proper handling of your ATV conducted through the Department of Transportation. Even if you think you know everything a human being should about ATVs, you might miss something important, it's good to have your child take a structured ATV class.
Most of all, make sure your child understands that a helmet is to be worn at all times. One person joked with their child the ATV wouldn't power up unless the ATV rider was wearing a helmet. The running joke worked as the child never got to wear their ATV helmet.
An ATV is a big step for your child, if they are ready and you know the size ATV your child is ready to handle, get ready for some fun on the trails with your kids!