Mouse might also be a bit of an unreliable narrator, especially with all that stuff about how great dogs are. That perception of the childhood monsters might be influenced by animalistic survival-of-the-fittest values a lot of writers give their intelligent animals (what with the whole predation can of worms they open up by making animals sapient), where every kind of creature has a place and a purpose in the natural scheme of things. It could also be, given the quasi-religious associations of his kind, he has a sort of religious-type world view, and that's his "problem of evil" explanation for them.

Maybe someone else might say that they are a kind of parasitical phylum of preternatural creatures who are so weak that they have to utilize a defense mechanism, like some sort of mystical thermal layer that allows them to pass unnoticed, and only kids and some animals have the capacity to perceive them, that adulthood and increasing will and intellect give you enough power over them that they become beneath your notice. Maybe it's something like the rules that constrain the Fallen and Outsiders, or maybe the childhood monsters have simply evolved to take advantage of a blindspot in the way the magical world & mystic perception works, like maybe they found a magical frequency on which to operate that is imperceptible to mature entities.

My interpretation of their ability to affect adults is that it is only to the extent that adults' behavioral or mental shortcomings permit. That they make themselves vulnerable by behavior that is immature to a degree, the way cheating on your spouse makes you vulnerable to a White Court vampire or a failure of faith leaves you unable to use its power against a Black or Red Court vampire or breaking an oath on your magic weakens your power or abusing the Swords of the Cross cancels their powers... there are lots of precedents in the DF world-building for something like that.