Interestingly, this short poem sounds to be more like the capturing of a moment than a note on motherhood in general. As a result, I think you could argue that Anne Stevenson means to to speak of motherhood through this poem as a collection of moments. In this case, images of Fall are used to show motherhood as something that is as natural as seasons, grows and changes, and dies too quickly.
The first two stanzas use images of a common walk in the park. The mention of the "light and dark" caused by trees and shadows paints a clear contrast between the youth of her daughter and the age of the "shriveled women" on the "cold benches." There is a feeling that death is nearby. In the same way that Fall gives way to Winter, this little girl will eventually grow old, as the speaker herself and then as the older women onlookers.
The final stanza seals the comparison of youth and age with the lines "you come so fast" and "you violate the past." This little girl is the reminder of youth and, perhaps worse, the quickness with which life seems to pass. The speaker has a view of the bigger picture of the seasons of life. It seems she is watching her daughter as the little girl she once was and watching the older women as the woman she will become. There is a quaint beauty in the imagery, but a certain sense of sadness in the diction. Perhaps the resulting message is that motherhood, in a moment, can bring together all the seasons of life, and serve as a reminder that it really is far too short.