Why has the poet brought in the image of merry children “spilling out of their homes”?

The poet harps on the introduction of the morose old age that is soon going to creep into her mother. She is recollecting the passage of time through images of “sprinting” young trees and the image of merry children “spilling out of their homes” is a proper depiction of the liveliness and ecstatic energy that adulthood holds in it. This is in sharp juxtaposition to the melancholia and decaying effect of old age which is represented through the image of the pale “dozing” face of the mother.