i think that she went to Bangor with Roger one day to get a Certificate of Governing. Although, I'm surprised that Elizabeth didn't take into consideration all the times Maggie was stolen away in the middle of the night and hidden in crypts, caskets, secret passages and basements for days on end. not to mention that screechy panicky voice she used to get all the time when something was happening to her.
I must say, though, that Maggie had a backbone when it came to the kids, especially david. she took no shit from him. but then again, he took none from her too and used to always mess with her head.
but seriously folks, there was a reason, i think it was because how maggie interacted with the children, especially david that elizabeth thought it would benefit all involved. i think that mags even tried to back out of it, and liz was adament.