My family have a habit of giving silly, rhymey nicknames as an affectionate thing. Some examples:
me,
Christine:
Kris Kringle
brother
Steven John: Steva
Johnnasister Elanor: Ella-Nella, Elle-Nell, Ella-Nella-Noria
brother
Robert: Robby-Bobby
cousin
Sonya:
Sonya Bonya (shortened to S.B. or
Bonnie)
I don't think there's anything wrong with it as long as the parents stop using it if the child suddenly decides that it affronts their dignity (8 or 9 is the usual age!).