There are numerous quads (four-wheeled ATVs) available for kids, you just need to narrow down how experienced the child is riding it, and exactly what you want in a 4-wheeler to make a good, safe choice that will make you both happy.
If your looking to purchase a quad for a first time rider. You should really look at the Kazuma KMX - 50cc. It has a very quiet engine at only 35 db which is equivalent to a soft whisper at a distance of two meters. A quieter engine means more safety for your child as they can hear other riders and other noises more clearly. Another feature of this 4-wheeler that makes it fantastic for first time riders is the governor (speed restrictor) that can be put turned on which cuts the maximum speed from 45-50 mph down to mph. This will lessen the chances of the 4-wheeler getting away from the rider while he or she is learning proper handling. There is also an auto kill switch. You simply attach the cutoff attachment to the back of the 4-wheeler and if your child needs assistance and you need to turn off the engine immediately, simply pull the cord and it's done.
For a more experienced ride, a quad that promises a lot of fun for your child is the Yamoto 110cc 4-wheeler with a sleek raptor style and an electric starter system, this is a very entertaining ride. As with all Yamoto 4-wheeler bikes, this one comes with a speed limiter screw so you can adjust the speed to the safest maximum speed your child may travel when riding their 4-wheeler.
If you're in the market for a quad, make sure your child has up to date safety gear. Children grow quickly and sometimes outgrow their safety equipment, always make sure your child has proper fitting safety equipment from head to toe. 4-wheelers are a lot of fun and being safe makes it an ever better time.