So, according to Our World and the DV in general, Fae have a lot of reasons for hanging out in the mortal world: Reasons as trivial as boredom (Real boredom, the kind of mind bending, nothing ever changes-in-the-Nevernever) kind of way, a desire for more earthly pursuits to direct needs in the mortal world (Leanansidhe drinks human blood from artists and poets in exchange for inspiration for them, drawing power from it). Humans are influences into the mortal world, a way for the Fae to amass more influence ('Oh, you want the city to bulldoze this park that happens to have ties to the Summer Court? Why, it just so happens I have this congressman in my debt... ') and a certain cachet. (If you're responsible for bringing in a Changeling who goes on to do something significant, that reflects well on you, which amasses power in the Sidhe world figuratively and literally) Lastly, the fastest way for either court to replenish or increase their ranks (and power as a result) is Changelings. That alone explains why both courts are inevitably very interested in anyone who's a Changeling. Depending on the Changeling's progeny, it could be VERY interesting.
The easiest way to get 'Mom' to leave them alone is straightforward: She owes the benefactor a favor (binding in the Faerie world). Alternatively, the benefactor owes the mother a favor--and if that's the case her leaving the child alone is a part of that. Generally speaking, with Fae plots, go complicated and convoluted: It just isn't all that fun for them if it isn't confusing and contradictory.
The obvious example of a kind of daisy chain of 'Why do they get so involved in Mortal affairs anyway?' Margaret runs into Lea somehow, manages to get a favor owed to her (Can't wait to hear that one!) That favor means looking 'after' Harry (and in typical Fae fashion she has a REALLLLLLLY funny idea of what that means). Lea's queen finds out about it, and finds a way to possibly earn favors and influence over a pivotal Wizard with some powerful connections and enough raw power to make a potent ally or servant. Because SHE's involved, Summer has to get involved, largely to stop that because there's no way in hell Winter gets an asset that powerful--but nobody gets to move openly.
And yes, it can all boil down to something as simple as care for her child. Though to make the story more interesting, it should be an alien thing, barely understood by the Fae mother in question--and turn out as well as a five year old who Reallllly loves her pet hamster (and very probably ends up accidentally squishing it).
