Next to the ATV itself the most important piece of equipment you can buy your child is an ATV helmet.
Both Thor and AFX make DOT and Snell approved ATV helmets. They also come in a large selection of colors and styles. When choosing a helmet, it should fit snugly that not too tightly, and should not move from side to side.
The helmet, which has a lot of padding inside, should stay in the position in which you place it on your child's head. Do not use old motorcycle helmets or even an old ATV helmet. The reason for this is that a motorcycle helmet won't offer the complete protection needed for riding an ATV and a helmet is good for one impact only. If the helmet has been in an impact or has been dropped from a significant height, something as simple as rolling off of the top of the refrigerator where it was just placed "for a moment", that helmet can not protect your child's head as there are most likely microscopic fissures and breaks and the ability of the helmet to protect the head has been compromised.
Most of the modern design helmets are a lot more lightweight than ATV helmets made even a few years ago. When your child wears an ATV helmet, make sure that the helmet is not so heavy it will cause fatigue in your child, fatigued riding of an ATV is dangerous to both your child and other ATVers.
A Thor or AFX helmet will offer years of fun for your child and with all the cool designs they come in - there is bound to be one that will fit not only your child's head, but their taste as well.