Sad for sure. I feel for the families in this tragic accident. The way I see it there is only one cause for this accident, poor judgement from a 16 year old.
I have run Fire and Rescue for 15 years and "most" crashes where I have had to extricate people and bodies from vehicles could have been avoided with better judgement, whether it be driving defensively, driving slower, driving sober etc. it's all a judgement call.
It's not the parents fault, it's not the cars fault it is the poor judgement on the part of the 16 year old. This kid decided to push his foot to the floor, probably showing off to his friends. The fact that he ignored the rules by having the other kids in the car speaks volumes about his ability to make good decisions and follow the rules.