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.