I've created a basic child image for twentytwelve, and it works as expected. The only issue is that it does not have a thumbnail image in the themes page:
How do I set a thumbnail image for my child theme?
Create image file in PNG Format having name screenshot.png and save it in themes root folder.The recommended image size is 880x660. Even recommended image size is 880x660 though it will only be shown as 387x290 but the double-sized image allows for high-resolution viewing on HiDPI displays.
For more information see this page.
If you don’t already have a screenshot of your theme, You can create one easily by following below steps:
What Vinod Dalvi said, except:
Make sure the file is named using all lower case: