The short version: You should upgrade Twenty Twelve to version 1.5. You currently have 1.4. Because you’re using a child theme, your changes will be safe.
The long version: In version 1.4, the JavaScript that controls the menu toggle for the mobile menu looks for an <h3>
tag to toggle the menu. In version 1.5, this was changed so that the JavaScript looks for a <button>
tag instead. It looks like your child theme was created with version 1.5 in mind.