I think there is something we are all forgetting... Harry has an amazing gift for influencing others both with words and deeds.  We remember Lasciel’s shadow eventually evolved into Lash which in turn later begat Bonnea.  So he has shown he can create life, a positive one at that, from an otherwise dark being. 

Mab, as much as any Mother, is sentimental.  Proof in her reluctance to act against Maeve directly and having Harry, as her Knight, move to counter Maeve’s attempts to destroy the Well.  Other interactions with Harry have led to glimpses in Mab softening just a little... being a little more open to kindness in lieu of coercion. I think Maggie’s gift is a reflection of this... I think Mab is, in her own way, being kind to his vassal’s daughter...Maggie is a reminder of her own experiences as a Mother.  Also due to the very nature of the Rites invoked to become the Winter Knight... Harry could be viewed as Mab’s Consort/spiritual spouse.  It would therefore make absolute sense for Mab to dote upon Maggie. Yes, Mab could act from an ulterior motive... but maybe, just maybe, even she recognizes it doesn’t profit her to have Harry maintain a constantly adversarial relationship.  Maybe giving Maggie a gift is her way of fulfilling her obligation to her vassal to be a worthy Liege as well as just repaying the minor kindness that Harry tends to give any female.... but at times even the courtesies he gives to Mab are not solely out of a Vassal’s obligations but from a more compassionate place in his heart.  He has in private lent his ear to her, particularly involving sparing her pride and grief over Maeve’s death.  That is something Mab may not find with anyone else.... and may also contribute to why she has finally found a Worthy Knight in Harry.  Just my thoughts.... I don’t think we have to automatically ascribe malice or manipulation to Mab’s actions in this case.