I know I'm super late on this, but just in case anyone reading this is worried about accidentally leaving their kids in the car a friend of mine figured out a great solution.

Get a long leash and pull it through part of the carseat and pull the clip end through the wristhole so it's a loop and it is now tied to the carseat. Use the clip and clip it to your belt loop on your pants, or purse or whatever is attached to you. Now you can't get out of your car without your kid.

She swears by it and made a habit of clipping herself after she puts on her seatbelt. It's kinda silly, but thought I'd share. It's cheaper than buying a new car with backseat alarms and not as irritating as taking off a shoe,