Yes we'll walk with a walk that is measured and slow, And we'll go where the chalk-white arrows go,
These lines are a repetition, with slight variation, of lines 10-11. The differences make it seem like the speaker is even more determined to get to the place where the sidewalk ends.
For example, the line begins with an emphatic "yes," and instead of saying "we shall," the speaker says "we'll," which is an abbreviated form of "we will." We're left with no doubt about whether or not this journey will happen.
And just in case you're still a skeptic, the speaker says that instead of just "watch[ing]" where the arrows go, "we'll go" where they go.
So, if we were unsure before about heading out of the city to a more magical place, now, we're absolutely gung-ho. We're definitely going to go, taking our sweet time, following the signs left for us. Come along, and invite your friends, your uncles, your dogs, and even your enemies to enter a world where all your troubles will surely disappear.
Finally, there are some interesting things going on with sound here. See how "chalk" in line 14 falls almost exactly where "walk" falls in the line before? This is something we call internal rhyme, because instead of rhyming at the ends of the lines, the rhymes fall somewhere in the middle. See if you can spot other places this happens in the poem.
For the children, they mark, and the children, they know The place where the sidewalk ends.
These lines, like the last two lines of the second stanza, repeat the central idea of going to the place where the sidewalk ends, and give us a new detail about just how to get there.
Our readers who are children can rejoice here. Finally, someone is listening to them!
In fact, they seem to know a whole lot more about how to get to the sidewalk's end than anyone else. The children mark what we might guess are the chalk-white arrows, because they know how to get to the place where the sidewalk ends.
Note the repetition of the word "children." This is kind of like the repetition of the word "walk" in lines 10 and 13. It really beats home the rhythm, and it also emphasizes the fact that children are the resident experts when it comes to escaping the paved world.
To truly understand these lines, we have to go back to the metaphorical meaning of this poem, because we're guessing that these children would get in a bundle of trouble if they actually went wandering out to the outskirts of the city, marking chalk arrows behind them. Their poor parents would have a fit.
So back we travel to the idea that the place where the sidewalk ends is inside our heads, in the wild world of the imagination. We can find ourselves a little strip of grass between the sidewalk and the street, and imagine that we are out with the moon-birds in the peppermint wind.
And really, folks, who better to show us how to imagine than children?