When you leave the house you check that your keys are present in your pocket/purse/backpack/etc.

When you leave the car you need to check that your kid is not present.

A simple solution would be to put whatever it is you require at work, such as your wallet/backpack/briefcase, right next to your kid in the backseat. You should check for your wallet/backpack/briefcase before you leave the car, which in turn will have you check the presence of your kid.

If you get your baby an android phone, at least you can use the Locate function to find out where you left them...

I've thought this too, something with an NFC connection being broken maybe. But then I think about the lawsuit that would probably ensue the first time it didn't work...