you haven't experienced the full list of variants yet - prepare yourself for
Smegs
Smeggles
Meggymoo
Mig
Midge
Mugs
Emm (plus whatever her surname.middle name is, so it could be Emem, Eminem - with the internal logic of Shady for that one - EmBee, EmCee, or EmBAH, etc)
And mispronunciations, such as Meegarn, etc.
Or she could do what a lot of girls do as they reach their teens and decide they want a completely different nickname, such as Bunny, Kitten, Harley (if Harley Quinn is back in fashion by then) or use her middle name/something based upon her surname.
Just chill. She'll always know her birth name, whatever she ends up being known as to other people.
(Caveat: I was given a name which is spelled differently if it's pronounced the way my father - Scots - said it. So, if my name's spelled 'correctly', it's not my name when spoken, if it's pronounced 'correctly', it's not my name when written down - as soon as I was old enough, I stopped using the fucking thing and went for a diminutive. Which then got lengthened by some. I don't care, as long as nobody uses my registered name, as I don't recognise that as meaning me.)