If it does not need to be portable, just find a source in the car that's not turned off would be my suggestion.
maybe a simple 'low-tech' solution is the answer,
i want to make an alarm to remind me and my wife to not forget our baby in the car (not planning to but these thing happen).
Hi Paul, fwiw, I originally thought the same, who could forget a baby, but apparently it has happened with disastrous consequences. (do a search of 'baby left in car') Although I think it is extreme to want an electronic device to remind you, I bet those folk wish they'd been reminded. The problem becomes, however, when you rely on a gadgets, and it fails, or is not quite showing things relevant at the time - e.g hgv drivers following satnavs, etc.
Then I would go for simple NiMH batteries. If left in the heat for long times they might not life as long but they are not as critical as a Li-type battery. They are used in solar lights all the time which get blazing hot as well.
Significant enough of a problem that there has been legislation proposed to require alarms in new cars Hot Cars Act of 2019