You can create a map where you provide your contact details. Then, click on the blue “Share Map” button and select “Link to Map”.

In the sharing dialog, you can download the QR code.

Print the QR code and stick it to the back of your child's smartphone or place it inside the phone's case.

If your child loses the phone, the finder can contact you through this method.

Note: This has actually happened in a similar way, and the boy got his phone back.