I need to write a letter based on this poem?

I don't want to discourage you, but the child is three and really wouldn't have that kind of epiphany. Perhaps you could write about how the speaker realizes that we humans have no control of nature and that even nettles...a simple plant...has power over other forms of life. We can't take any form of nature for granted. The sea may be calm and beautiful one moment, but can crush a ship during a storm. The sun may be warming and nurturing, but can burn and damage. The parent in the poem will try throughout the child's life to protect him from many things, but can never really protect him from the forces of nature.