Did you happen to add some customization for the menu area through the child theme? It seems that the structure of the header is different to the default header structure. If you did, kindly try removing the custom code.
If you did not add any code, please try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If the issue persists, please provide us with the admin access to your site in a Secure Note: