Reddit reviews Asking the Right Questions: A Guide to Critical Thinking (8th Edition)

Hi TruthToPower1, you might find this useful.

This one is a great start

UPDATE: This one is recommended right after the first one

I would go with something general-purpose like this (haven't actually read that book in particular myself, it was just the first hit that looked like more than pop fluff). Teaching them critical thinking skills will do more to encourage critical thinking than any amount of Dawkins or Hitchens (which are more aimed at people who already lean atheist, but want to refine their arguments/understanding), and a general text like that, that isn't explicitly anti-religion, is more likely to gain some purchase with them anyway.