Right, and that's a choice. It has nothing to do with "she's a 7 year old" and everything to do with "she's allowed to make a mess in the car".
I was never allowed to eat in the car beyond the odd sweet, and I wouldn't have thrown wrappers or anything else on the floor on pain of death... I find it mind boggling that the child is blamed for the state of the car, when realistically it is just a parenting choice. There are 7 year olds with exceptional table manners even in this modern day, and there are 7 year olds I'm embarrassed to eat at the same table as.
I'm fine with people choosing to have messy cars (mine oscillates between spotless and a dump over ~3 month periods) - but it's a choice and other choices can be made.
Maybe it is "choice", but there are a lot of people out there where the "choice" around their vehicle is closer to that of it is lived in, may have books and papers, clothing, toys, etc. (maybe food as well) that will not conform to this scenario.
Walk through a grocery store parking lot sometime and observe the state of vehicles as people get in/out, etc. my bet is on a sufficiently large pool would be in the condition where utopian sharing doesn't make sense.
That said, if there was a mind shift because of an overall benefit, it could happen. But, that is a long way off.
A friend with an incredibly smart 7 year old still has a messy car. Her daughter, despite the ability to read and understand things well above her age, is a 7 year old. The car is a mess.
When it comes to these pie in the sky utopian scenarios, I wish people on HN would step outside and actual observe every day people.