And we'll go where the chalk-white arrows go (14)
These chalk-white arrows appear, though we're not really sure where they come from. They take us to the place where the sidewalk ends, and we're expected to blindly follow them, without asking any questions. But, remember, we're no longer in the real world. In this imaginary world, strangers give us candy because they're just being nice, and directions magically appear in chalk to take us to the places we most want to go. Lucky us.
For the children, they mark, and the children, they know (15)
This line gives us the idea that children are the best equipped to create marvelous worlds in their minds. They haven't had day jobs or had to scrounge to pay the rent, they have never driven a car and maybe never traveled very far. Yet it's their unfettered imaginations that can take them to worlds that some adults can no longer access – like the magical place where the sidewalk ends.