Having recently re-read Foxglove Summer I feel more confident turning the previous comment into an answer.
<PERSON> is a man with fidelity issues. At approximately the same time 11 years ago he fathered children with his wife, <PERSON>, their babysitter <PERSON> and <PERSON>.
<PERSON> is the simple one, she's <PERSON> and <PERSON>'s child. Raised all her life by <PERSON> and her husband.
The child known as <PERSON> is the daughter of <PERSON> and the Fae. She is the changeling baby swapped by <PERSON> for her real sister at a young age.
"Not-<PERSON>" is the daughter of <PERSON> and <PERSON>. She's spent most of her life in the fae country and returns to Rushpool with <PERSON> half way through the book.
By the end of the book, <PERSON> and <PERSON> have been returned to the homes they were raised in and Not-<PERSON> has been sent to London to become part of Mama <PERSON>' responsibility as there does not seem to be any other way to deal with a human child with significant fae adjacent powers probably not limited to the strong glamour ability demonstrated throughout the book.
It seems likely the <PERSON> is only interested in children with some connection to her. Even if that connection is one step removed via the father of her own child. 247
So the changes of the Fae abducting an arbitrary child at the next full moon seems slim.
The second thing restricting the activities of the <PERSON> would appear to be their vulnerability to Iron and the existence of the Roman Road in both the real world and the Fearie Kingdom. The road provides an access route that is potentially vulnerable even to non-magical incursion. Which explains the care fae take when crossing it while moving <PERSON> around. <PERSON> has demonstrated the damage she can cause quite conspicuously by simply showing up in a steam engine and every sign we've been given shows the reluctance of the fae to submit to a repeat engagement.
Whether The Folly, or the Rivers are capable of causing as much damage to the Fae in general as the rescue of <PERSON> showed outside of those specific circumstances is left unclear to both the readers and the characters. Leaving us in an uneasy stalemate that may well contribute to future stories. The area is briefly touched upon in The Fey and the Furious. But it doesn't fully answer any of your questions. 45