My view is that kids riding bikes should wear helmets, period. (assuming they are exposed......the big car seat things for the really little ones may be the exception...but once there are beyond that, even if trailer helmets)

some stats from safekids.org

In 2001, nearly 314,600 children ages 14 and under were treated in hospital emergency rooms for bicycle-related injuries. Nearly half (47 percent) of children ages 14 and under hospitalized for bicycle-related injuries are diagnosed with a traumatic brain injury.

More than 40 percent of all bicycle-related deaths due to head injuries and approximately three-fourths of all bicycle-related head injuries occue among children ages 14 and under.

Riding without a bicycle helmet significantly increases the risk of sustaining a head injury in the event of a crash. Nonhelmeted riders are 14 times more likely to be involved in a fatal crash than helmeted riders.

Children ages 10 to 14 are at greater risk for traumatic brain injury from a bicycle-related crash compared with younger children, most likely because helmet use declines as children age. Helmet use is lowest (for all ages) among children ages 11 to 14 (11 percent).

Bicycle helmets have been shown to reduce the risk of head injury by as much as 85 percent and the risk of brain injury by as much as 88 percent.Bicycle helmets have also been shown to offer substantial protection to the forehead and midface.

It is estimated that 75 percent of bicycle-related fatalities among children could be prevented with a bicycle helmet.

From Journal of American Medicine

From 1984 through 1988, bicycling accounted for 2985 head injury deaths (62 per cent of all bicycling deaths) and 905,752 head injuries (32 per cent of persons with bicycling injuries treated at an emergency department). Forty-one percent of head injury deaths and 76 per cent of head injuries occurred among children less than 15 years of age. Universal use of helmets by all bicyclists could have prevented as many as 2500 deaths and 757,000 head injuries, i.e., one death every day and one head injury every 4 minutes

There are a lot more from diverse sources

I don't want this to get into big helmet thing, pleasesgo to A&S for that, and look for the first sticky , but IMO cyclehelmets.org is highly biased and should not be used as sole source for basing safety decisions.
At a minimum look at www.bhsi.org also. This is also a biased site....but their citations come from a much broader base. and they will note areas of disagreement.

I am also all for Adults making their own decisions and accepting the responsibiliy that comes with those decisions. , but remember monkey see, monkey do is the rule with kids.