What has the camera captured in "A Photograph"?

In Shirley Toulson's poem "A Photograph," the camera appears to have captured a moment shared by three girl cousins at the beach. They are on a vacation at the seaside, evidently. The older girl in the middle of the two younger ones is the mother of the speaker, and she is about twelve years old in the picture. The girls all "smile through their hair" at their uncle, the person taking the picture, and it seems as though they have been swimming. The photograph has captured the speaker's mother's "sweet face" as well as the sea, along with the girls' "transient feet." The girls could not stay little girls because we all have to grow up, though the sea has remained relatively unchanged in appearance.