def visit_children(parent)
      parent.children.each do |child|
        child.node_parent = parent
        visit(child)
      end
    end
def visit_children(parent)
      parent.children.each do |child|
        child.node_parent = parent
        visit(child)
      end
    end
Redefine so we can set the `node_parent` of each node

 @param parent [Sass::Tree::Node, Sass::Script::Tree::Node,
   Sass::Script::Value::Base]