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I agree with this response. If the content is mostly under your own control (the main content is yours, maybe with some scattering of UGC 'comments' from parents) then go subdirectory If in the future you create an area where you have way less control over what is posted (like a forum, which is 99% UGC) then go subdomain

I agree with everyone above. A great tool for this may be SEO Yoast Premium. When you create categories that changes your permalink structure it automatically creates a 301 redirect for each page.

Thanks Chris and Alex for your response. I think I should focus on building links to category pages and high traffic potential posts and let the page rank flow to other pages with less traffic potential.