Heatstroke?
“In the confined space of a car, temperatures can climb so rapidly that they overwhelm a child’s ability to regulate his or her internal temperature. The body, especially a small body, can go into shock quickly, and circulation to vital organs can fail.”
https://globalnews.ca/news/2142475/heres-what-happens-to-your-body-when-youre-left-in-a-hot-car/