The poem "My Father's Sadness" by Shirley Geok-lin Lim discusses how a father's responsibilities can be overwhelming, and how the responsibility of constantly caring for his children can take a toll on him. This is the theme of the story.
In lines 13-16, the author imagines that her father is made of clay. She says that with each child, a little bit of him is lost. "The clay of his body crumbling" refers to the father giving away a little bit of patience, energy, happiness, etc. to his children, and slowly decaying. These little pieces of clay are like the drizzle we can see in the hourglass.