If you're thinking of purchasing an ATV for your child who is over the age of 6, you need to consider a few things before jumping in with your purchase.
ATVs are a huge amount of fun, but riding them is a big responsibility and you must be sure that your child is ready to handle that responsibility for their safety as well as the safety of others.
Children mature at different rates, your 10 yr old may be mature enough to handle riding an ATV and their 12 year old cousin may still lack the maturity and good judgment to do the same. You need to look objectively at your child and decide first if they are big enough to handle an ATV.
ATVs come in all sizes, for just about any size and body type. Make sure your child is physically ready and they can place both feet on the foot rests, reach the handlebars comfortably and easily reach the foot controls. Make sure your child can squeeze the hand controls and operate the shift lever and brakes with ease.
Your child also needs to be able to read and recognize what the various signs and trail markers mean. Make sure your child has peripheral vision and knows to be aware for other riders. Your child also needs to be able to judge the distance of objects from his or herself.
You need to further ascertain that your child is emotionally ready to ride an ATV. If your child is easily frustrated or upset and acts out accordingly, it is probably best to pass on an ATV at this time. If you child can keep their mind on driving their ATV safely and keeping an eye out for others, it's probably time to let them enjoy the freedom and fun an ATV offers.