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.