The earliest version of this quote that I was able to find appeared on Twitter in 2009.
RT @iamsuperhero: we stopped looking for monsters under our beds because we realised they were inside us.
@tinkeringbeII, 2009-10-19 10:32. twitter.com.
Note that this is a retweet (from the days when retweets had to be done by hand), so the quote is older than this, but the retweeted account is locked, so I was unable to trace it further back. The quote became popular on Twitter: if you do your own searches, you'll find many hundreds of instances in many modified versions (“stop” for “stopped”; “the monsters”; “underneath” for “under”; “when” for “because”; and so on).
So I think that pending an earlier discovery, a likely origin is on Twitter, perhaps from user @iamsuperhero, or perhaps from another user whose account has been locked or deleted.
From Twitter the quote made its way to other social media sites and eventually into print. The earliest printed instance I can find is in Passionaries (2013), a young-adult paranormal romance by Tonya Hurley:
‘They say that we stop looking for monsters under the bed when we realize they’re inside of us,’ he paused. ‘Just as we stop looking for evil. There’s evil inside all of us, Cecelia. You are no different.’
Tonya Hurley (2013). Passionaries, p. 119. London: Hodder.
The quote was soon mis-attributed to Darwin. The earliest instance I can find is Charles Darwin: His Words (2014) by Daniel Coenn, a self-published e-book. A printed book with the mis-attribution appeared in 2015 (this is the one referred to in the question):
We stopped looking for monsters under our bed when we realized that they were inside us.
It is always advisable to perceive clearly our ignorance.
— from The Expression of Emotion in Man and Animals
Anon, ed. (2015). Darwin on Evolution: Words of Wisdom from the Father of Evolution, p. 46. New York: Skyhorse.
These collections of quotations are very quick and cheap to publish, and often contain revealing errors suggesting that they were copied from other collections of quotations, and not from original texts. They should never be depended on for scholarship or evidence. In the Skyhorse case, the quotation “it is always advisable to perceive clearly our ignorance” is genuine, but the book was titled The Expression of the Emotions in Man and Animals (not “Emotion”), suggesting that the editor at Skyhorse had consulted a secondary source like Coenn, and not the original work. Presumably the “monsters under the bed” quotation got in via a similar reliance on secondary sources.