Calliope is a long name--4 syllables--and it's very obscure.
Phoebe and
Daphne are all two syllables and fairly well-known. Almost all 4 syllable names (
Alexandra,
Elizabeth, etc) get shortened at some point. If you really hate
Callie, don't use
Calliope. I promise that at some point she will be called
Callie. Also, she may decide that
Calliope is too weird and she wants to go by
Callie.
It has nothing to do with you "letting" her. It's her name--she gets to choose.