Since I just had this whole back and forth with someone else a few days ago, I have these handy. I’m not the parent, but he’s right. An individual car can be more fuel efficient with 3+ passengers but the average car trip is only 1.3 passengers. The most popular use of a car is commuting and that stands at 1.2 passengers per trip.

“A new report from the University of Michigan’s Transportation Research Institute shows that flying has become 74% more efficient per passenger since 1970 while driving gained only 17% efficiency per passenger. In fact, the average plane trip has been more fuel efficient than the average car trip since as far back as 2000, according to their calculations.”

http://websites.umich.edu/~umtriswt/PDF/UMTRI-2014-2_Abstract_English.pdf

“The main findings are that to make driving less energy intensive than flying, the fuel economy of the entire fleet of light-duty vehicles would have to improve from the current 21.5 mpg to at least 33.8 mpg, or vehicle load would have to increase from the current 1.38 persons to at least 2.3 persons.”

https://yaleclimateconnections.org/2015/09/evolving-climate-math-of-flying-vs-driving/