We live in our own world,
A world that is too small
For you to stoop and enter
Even on hands and knees,
...
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The first few lines emphasis that the adults are unable to go back to their childhood. Even if you go on your hands and knees and try acting like a child, you still won't be able to be a child. The blue color of the birds' eggs are a brighter blue than the sky itself. The eggs are 'mocking' the polluted sky. The blue sky is a metaphor for happiness, which has faded and become remote.
The poem is about us and what we have lost.