Over the last year or so I’ve had to create Child Themes for testing purposes. Using the Official Magento Documentation (that I recommend you read first), I created the below Starter Files.
I don’t recommend blindly uploading them to your project without first taking the time to learn and understand how it works.
If you have any questions, please refer to the Official Magento Documentation before starting a new topic.
Disclaimer: These appear to have worked when I last checked with both Magento 2.2.x and 2.3.x.