What does the poetess say about the sea and the feet of the children in the photograph?

She says the human's feet and bodies are always changing, they are fleeting and impermanent, while the sea always looks the same and seems to look unchanged.  

Explanation:

Poetess Shirley Toulson expresses her experience of watching a photograph of her mother as a child in her poem "A Photograph".

In one part she describes the image with the words:

And the sea, which appears to have changed less,

Washed their terribly transient feet

This line reflects the passage of time and the stillness of nature.

The feet of the girls, one being her mother as a child, have changed so much over the years. The girls got older and their bodies grew older, so the feet so have changed so much and to exists like that only for that short moment of taking a photograph. The look of the human’s body is only temporary, it changes until it finally dies and disappears from the material world, just like the poetess’s mother did.

Yet, nature seems to stay the same – seas stay in the same place for hundreds of years, always remaining blue. No matter how many people pass, how many people it touches, it looks the same to us.

Therefore, the poetess compares the passage of time and how it is reflected in the human body, to how it doesn’t mean anything for nature.