Theory:

The poem has two scenarios; the child is on the way to school, and the child is at home. In these two scenes, the child saw three people in a day. The poem conveys the child's mindset and the wish towards the people who hide their struggles and pain behind their works.

One morning, when the child rushed to the school, the child saw a hawker selling bangles. In the evening, the child saw a gardener digging the soil, where he never bothered the stain on his clothes. At night, the child peeped out through the window and saw a watchman who will never sleep at night and enjoy the whole night with his shadows.

The child feels jealous of those people as they are happy without any time restrictions. From all those works of the people, the child wished to be a hawker, a gardener, and a watchman. He doesn't want to push himself to go to school on time and follow the time restrictions, whereas the child needs a peaceful life.
 
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Hawker, Gardener, Watchman