I would be extremely interested if you were to hypothetically write an “intro to child protection/welfare for EAs” post on this forum! (And it would probably be a great candidate for a prize as well!) I think the number of upvotes on this comment show that other people agree :)
Personally, I have ~zero knowledge of this topic (and probably at least as many misconceptions as accurate beliefs!) and would be happy to start learning about it from scratch.
“Cause X” usually refers to an issue that is (one of) the most important one(s) to work on, but has been either missed or deprioritized for bad reasons by the effective altruism community (it may come from this talk). So I’d expect a cause which the EA community decided was “cause X” to receive an influx of interest in donations and direct work from the EA community, like how GiveWell directed hundreds of millions of dollars to their top charities, or how a good number of EAs went to work at nonprofits working on animal welfare. (For a potentially negative take on being Cause X, see this biorisk person’s take.)
I’d also be very interested in a book review post of The Body Keeps Score. (I may do this myself at some point, but not sure when I’ll have the capacity.)