C.C.
Cara is a beautiful name, but I have to ask - have you considered the possibility that "Cara Ball" rhymes with "terrible" (and wearable, shareable, etc...maybe even hairball) - I'm just putting that out in case you had not considered it, in terms of schoolyard nicknames.
ETA: maybe "Melinda"? Then she could be "Cara Mel" as a nickname? "Cara Mel Ball"....