Lines 21-25 makes all the difference when you look at the theme of the poem which speaks to Children Growing up and eventually leaving home.
Explanation:
The learning how to ride a bicycle is a metaphor for life.
Every other line seems to be talking about her eight-year-old daughter learning how to ride a bicycle. See lines 1-6.
Her surprise is ignited when her daughter starts "pumping away" at the pedals screaming with life. She is surprised to know that the mother-daughter experience as she knows it is over already. See lines 7-10.
But when you read those lines "like a handkerchief waving goodbye" all of a sudden the entire picture, the intended message hits you and you are almost moved to tears when you see from a mothers perspective how ti feels to quickly a daughter grow up and leave home.
