Rather than a car feature, what about one of those sensors that alert you if the object is too far? And force the user to explicitly acknowledge that it's intentional (for when you leave them with someone else).
How do you reduce false reports (e.g. dropped connections) enough that people won't shut it off in annoyance, though? For a technological solution to really work for the "break in routine makes someone forget a baby in the car" case, it has to be something that can be left in place forever once activated.
Good question. I wouldn't warn on a single broken connection, only after a couple of minutes without a signal, but I suppose even that could be annoying (e.g. kid sleeping on the next room).
