code: function children(selector) {
    var arr = [], slice = this.slice, nodes, matches;
    this.each(function(el) {
      nodes = slice.call(el.childNodes);
      // only include elements
      nodes = nodes.filter(function(n) {
        if(n instanceof Element) { return n;}
      })
      // filter direct descendants by selector
      if(selector) {
        matches = slice.call(el.querySelectorAll(selector));
        for(var i = 0;i < nodes.length;i++) {
          if(~matches.indexOf(nodes[i])) {
            arr.push(nodes[i]);
          }
        }
      // get all direct descendants
      }else{
        arr = arr.concat(nodes);
      }
    });
    return this.air(arr);
  }
docs: Get the children of each element in the set of matched elements,
  optionally filtering by selector.