do NOT let them sleep in their car, there's a real risk they could die if from asphyxiation. Here in Mexico there were blackouts from the record heat and there was a family who slept in their car for the ac. They unfortunately all died
Assuming they're sleeping in the car with the AC running, my car never seems to let me keep it on recirculation for more than like 10 minutes. I noticed this while I was driving through a bunch of forest fire smoke I was trying to keep out, and I had to keep turning it back on.
Maybe that's just a safety feature my car has, but not all cars? Or am I misunderstanding and it's an issue with exhaust fumes? That would definitely be an issue in a garage.