Piper wasn't really sure how Phoebe had ended up as being known as Kiddo, it might have been Grams or Andy or Prue that had started calling her younger sister that but somehow it had stuck, even into adulthood. Even now that she wasn't the baby of the family.
Piper could remember a million and one times when she herself had called Phoebe that, and then another million others of everyone doing the same.
So as she found herself sat in the rubble of their home clutching at the body and begging for her baby sister to look at her, to just breathe. She called her it once more.