I would go with subpage, but the corrector on stackexchange thinks it's a mistake and shows a red underline. You can try it yourself. Click on "Ask Question" link and type subpage in a sentence to the body textarea.
Why is that? I don't understand. Is subpage a word? Because according to this:
It's not even a word!
I don't understand it at all. I and millions are using this term everyday.
Could somebody, please, explain that to me?
Btw. Microsoft is using "subpage" word for Office products instead of "sub page": http://office.microsoft.com/en-in/onenote-help/about-pages-and-subpages-HP001112525.aspx