Child theme support question

Hi,
I use a child theme and I’ve made a child theme for mobile and selected it in your plugin. Am I wrong and just not understanding something, or can I not create a custom page-[ID].php template for a page that already has a custom file on the main/desktop theme? I can create a page file for a new page and that works fine, but the page which already has a custom file on desktop isn’t displaying – it show’s the content of the WP entry.

Thanks