Hi Andrew, thanks for your advice, got me thinking about it. I've come up with this:
<r:unless_children><r:comments:all /></r:unless_children>
This satisfies my requirements:
1. I want to use the same layout for the parent and children pages for user convenience (i.e. inherit layout when creating child page).
2. The parent page is not the root page.
3. I only want comments to show on the children pages
Thanks again,
Dean