I have a child who does not sweat and quickly overheats. It wasn't easy but we managed. Guess how long we managed? 4 YEARS. She was rear facing for four years, 4 seperate summers.... In a Marathon/Wizard/Boulevard for the record.
My child literally is beet red after 10 minutes in 85 degree weather. Just BEING outside, not even running or anything. She can go outside and play for a few minutes, then has to come inside to cool off for 20 minutes or so.
This was within a maximum of ten minutes at our family reunion. It was 85 degrees at the most and she was under a pavillion.
We had the air conditioning on full blast, all the time. It was not uncommon for me to wear a sweatshirt in July, just to keep her cool enough. She drank lots of cold water in the car. If possible, we drenched her when we got to our location. Or while we were there. Like at the aviary the other day, we drenched her with the drinking fountain every ten minutes. Get her face wet, just let her play in the water. It's summer, it'll dry.
There is often a way around the "inconvenience" of keeping your child safe, the parent just has to care enough to find them.