This entire project costs less than $10 and can even be free if you already have a greeting card with audio.
All you need is an audio greeting card (with or without recording option), string, tape, and a small piece of paper.
As a college student, I don't have access to a car at the moment, so I decided to test the project on my door and possibly scare my roommate a bit in the process.
Disclaimer: This project makes no guarantees about successfully reminding parents about children left in their car. The device gives an audio reminder which may be ignored or the device may not have been set-up to begin with. This device is only meant to be an additional reminder.
Step 1: Record a Message
My recording welcomed people to my room, asked why they were doing, and eventually asked them to leave.
Step 2: Open the Card
Observe how the card works by opening and closing the card a few times. Opening the card allows the contacts on the pcb board to touch and sets the card off. We'll simulate this with a piece of paper between the contacts that is pulled away so the contacts can touch in the next step.
Step 3: Attach
Tape or glue a small piece of paper to the end of the string. This will serve to separate the contacts on the card. It is convenient to cut a triangular piece of paper (I just ripped off the corner of the envelope the card came in) as you can slide the piece of paper underneath the tape sticking the string to the door when you're not using the device (as shown in the picture).
Tape the other end of the string to the moving platform.
Tape or otherwise attach the rest of the card to the moving platform.
Place the piece of paper between the contacts to set up the device. You'll need to reset the device by doing this every time you want the device to go off.
If you want to leave the device long periods of time then remove the string from the moving platform and leave the paper between the contacts, so that you don't drain the battery.