Additionally children also have a less develeloped cranium. The arguments for adults and children are totally different in this aspect and cannot / should not be interposed.
Having said that.............
The only crossover point is the perspective that cycling is dangerous enough to wear a helmet when the more common causes for head injuries are considered a fair risk.
If you look at the cause of head injuries in children only 20% are cycling related - why are all the iresponsible parents not making their children wear helmets during the activities causingthe other 80%?
(Figures from NHS Direct)
As for shopping...................
link
In the States 17,300 children under 5 suffer head injuries in falls froom shopping carts - do we see helmets having a contribution here?
Will yur child be wearing a helmet nexttime you o to Tesco /
Morrisons / Safeway?