In Michigan it is illegal to ride double on an ATV ( http://www.michigan.gov/dnr/0,1607,7...ml#Regulations
see "Other Operation Regulations" )
I agree with this law in that ATVs (not designed for two people) cannot respond correctly and will tip easier at any speed with two people on it. Also it is not JUST the helmet to be concerned about here. The rider cannot "bail" as quickly (or at all depending on position) and all persons involved are at risk for greater injury. The driver is because he now has to worry about hurting the passenger (landing on, rolling over, crushing underneath) and the passenger has to worry about how to get out of the way (no proper handholds or foot rests to push off from, wrong body position for quick exit, trapped under or around the driver).
An ATV (not designed for two people) is unsafe all the way around when riding double. So yes... your kids may love it... Yes ... You have not been hurt so far... and Yes .. it is nice to take your children with you. BUT is it worth a life? The helmet will protect the head but a childs rib cage and abdomen cannot withstand a crushing blow of an ATV and maybe the driver also. This is another reason why age size guidelines were created. Larger people larger quads, smaller people, smaller quads. Yes little Johnny can make the Sport Quad/Farm Quad go down the trails and also be legal BUT does he have the weight to control it or the body mass to withstand the impact when he doesn't?
Give you a little story (or two) or a grandfather who wanted to take his grandson out for a fun ride. Went up a little hill and the ATV rolled over backwards (center of gravity moved back due to second rider). Grandfather landed on the child, ATV landed on grandfather. I am not sure that killing his grandson was what he had in mind.
Second story... me a few years ago and a cute girl who wanted a ride. Hey,, who can refuse a cute girl? Same thing happened. Went up a small grade that I have been over thousands of times and up come the front end (could not slide forward as she had me around the waist) and over we went. She hit the ground, I hit her, lucky me the ATV missed us because she had landed on a small stump and it would have punctured her side if it had not. I am glad she did not get hurt but it was a scarey lesson. You CANNOT react, control, or balance correctly due to the second rider.
There... My 2 cents are in.
p.s. I have been riding for 29 years and so far no broken cranium...