There is a place where the sidewalk ends And before the street begins (1-2)
These lines talk about two very important settings for childhood: the sidewalk and the street. The sidewalk and street may both be in sight of children's parents' watchful eyes. So, of course, the idea of escaping to a place where they can be totally free might be appealing.
And there the grass grows soft and white (3)
Anyone who has cleaned a grass stain off a pair of jeans, or seen a child giggle with glee as they roll down a hill, only to complain that they are itchy and scratched up, can understand the significance of this line. Children love to play in grass, but grass in the real world isn't always super pleasant. So a land with grass that's soft and white – no stains or scratches – is a blessing for kids and parents both.
We shall walk with a walk that is measured and slow (10)
If you've ever taken a long walk with a bunch of kids, you'll know that they are likely to run off with a giant burst of energy, only to quickly become exhausted. So it's often the parents who have to come up with games to keep their children contained and controlled, like counting steps. That's what this line reminds us of: the children are destined for an imaginary place, but they're going to have fun on the way as well.
And watch where the chalk-white arrows go (11)
Chalk is something that reminds us very much of children, and one of their favorite ways to have fun on the sidewalk: making maps and pictures and drawing hopscotch courses. Following a path drawn out in chalk would be just the way to enchant a child into imaginative exploration.
For the children, they mark, and the children, they know The place where the sidewalk ends (15-16)
But there are a lot of things that small children know that adults may have long forgotten – like the wonderful places an active imagination can take you. So at the end of this poem, we have to seek out a child to take us back to the land made of dreams.