IIRC, this is when he's referencing Where the Sidewalk Ends, which is a metaphor for the shift from the carefree nature of youth and the responsibilities of adulthood, and how every child understands there's a moment where they'll have to grow up.
Adulthood is portrayed as a dark and scary place, and by contrast youth is seen as bright and fanciful. And the poem encourages adults to view life through the lens of a child to find the joy in the world -- which is essentially what Sensei is doing.
Not spoken within the poem is the inverse of that -- children viewing the world the the eyes of an adult and feeling the existential dread. Which is what some of the girls are starting to do -- Makoto in particular, but a few of the others have as well.
I'm not sure what the guard rail reference is. I don't recall any references to one in any of his poems.