My heart goes out to the mother of this child. Just one questions I have why was her not wearing a helmet.

Hopefully other "parents" will learn from this tragedy. TEACH your children about obeying traffic laws, signs and signals, before you just allow your kid to be out flying around in the streets with no training and no safety equipment.
I'm not saying that was the case here, but I do see plenty of youngsters who are completely oblivious to traffic laws and are probably too young to be riding in the streets and sharing the road with vehicles that they have no chance against. Perhaps there should be an age requirement for a young person to be riding a bicycle on the road.
My prayers are with the family in their time of loss.

i almost never see any children in the city wearing helmets unless they are very young, just learning to ride (on the sidewalk or in the park) and their parents are with them. by the same token i often don't see parents with them on when their children have them on. today i saw a woman with one of those pull behind carriers riding her bike in town and while it was nice that she was taking the kid on an outing and he had a helmet on inside the carrier, she did not. so i think it is a matter of example too.
i am not blaming the parents though, i want to make that clear. kids often leave the house without doing what they know they are supposed to ("do you have a coat?"... "yes mom"... you know, that sort of thing).