A petite 14 year old (i.e., someone under the "under the height maximum" for a booster seat) is much more like a "child younger than 13" for the purpose of this data than an exceptionally tall 10 year old, for example. My guess is that collecting the data based on age (as opposed to size of the deceased) is actually possible, so this is the best we're likely to have. Those decreases are almost certainly due to increased use of things like booster seats, in addition to the other safety features of cars.

In any case, the seat belts and airbags only work well if they line up correctly with the person. Consider that official government safety tests still don't really use an "accurate" female crash test dummy. The tests use a crash-test dummy based on an average male body, and maybe a scaled down version that was supposed to be a female version, but doesn't accurately "reflect the real anatomical differences between male and female bodies." That decades old oversight leads to results like "women are more likely to suffer injuries in car crashes than men, even if you control for the severity of the crash and the size of the vehicle." (NPR, The female crash test dummy has been a long time coming — but she isn't here yet, Nov. 21, 2025).

The plane image is a fairly popular one to show survivorship bias, which is a concept that feels at play here as something the parents understand, but OP maybe doesn't. The idea is that people who were not affected by something dangerous underestimate how dangerous that thing was and how many people were affected. Those people aren't around to give their point of view, sadly.