On screen twice:
- She regenerated from being the little girl in "The Impossible Astronaut"
- She regenerated from being Mels to being River Song.
We do not see what form the little girl takes or the form that preceded Mels.
However we have have no information about what happened between the end of "The Impossible Astronaut" and "Let's Kill Hitler". It might be considered likely that she may've grown up and made her way to Leadworth before becoming a child again, it might've been difficult getting there as a child, or she may've been placed there again by the Silence, though she does indicate that meeting her parents as children was voluntary.