While
Maggie isn't the most common
Margaret diminutive for children right now (it's
Maisie) it's still far from unusual - and both
Maggie and Maisi are more common than
Margaret.
Maggie would probably sound more adult than child, but a
Maggie of any age wouldn't be too out of place.
William called
Will is more common than just
Will, but
Will is not unusual. There are Williams of all ages, but
Will would tend to be 40 and younger, while
Bill would tend to be older. (But oddly
Willie would be older than
Billy.)