... And to further complicate things (please correct me if I'm wrong, it's been a while since I've read Changes), but wasn't Maggie's adoptive family violently murdered? And then the child missing from the murder scene shows up in the US a few days later with no records of crossing borders? ...
I've got a strong suspicion that li'l Maggie was mostly off-record. I doubt she was even registered in whatever jurisdiction Susan placed her, let alone registered with enough info to link her to the little girl Harry calls his daughter, in the USA...
... Forget social workers, all someone would have to do is shine a light on how Maggie even got to the US in the first place, and Harry would be under immediate investigation from the FBI, CIA, and any other agency that investigates international crimes - for murder, kidnapping, and human trafficking at the least.
If they (whoever "they" are) could link the little girl from Latin America to the little girl in the USA, this could be a problem. Mavra threatened Murphy with purely-mundane legal threats, so others might threaten Maggie similarly...
But then again: Harry was in the USA when all this went down with Maggie's adopted family (the FBI interviewed him (repeatedly!); he probably has other alibis too).
Forthill has likely got a "true" birth cert -- true in the sense of listing Harry & Susan as parents (as they were; so she *IS* a citizen (and appropriately in Harry's care, no "kidnapping" or trafficking)). Maggie has Harry's features & Susan's, we're told -- strong family resemblance. If needs be, DNA proof could no doubt be arranged (despite the risks of spells).
So the link to a foreign crime-scene is dubious, and the proof that Maggie "belongs" in the USA (with her dad) is indisputable, despite any "irregularities" in her arrival. I bet Lara Raith would even cover that part of the paper-trail, simply as a favor for Thomas (who's officially in good odor with the House).
These efforts could only be a temporary inconvenience to Harry, at most. Still, a US District Court summons
could undoubtedly be managed for a maximally-inconvenient-time for Harry... And anything that even looked like a threat to his custody would put him straight into stress/anxiety/rage (and thus increase the likelihood of mistakes (but also thus fuelling his magic, so... not their best move!)).