There's a free app for iOS called iSafe Child, works by basically pairing any Bluetooth headphone, placing the headphone in your child's pocket, and works by alerting you (I guess vibrates) when the Bluetooth connection is out of range which is 20-30 feet.
Downside would be you'll need to make sure the Bluetooth device is charged, and all audio from your phone would be sent to Bluetooth headphones.
If you already have a set of Bluetooth headphones, you might as well give this a rest trial.
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