function getChild(el, childNum, options, includeDragEl) {
    var currentChild = 0,
      i = 0,
      children = el.children;
    while (i < children.length) {
      if (children[i].style.display !== 'none' && children[i] !== Sortable.ghost && (includeDragEl || children[i] !== Sortable.dragged) && closest(children[i], options.draggable, el, false)) {
        if (currentChild === childNum) {
          return children[i];
        }
        currentChild++;
      }
      i++;
    }
    return null;
  }
Gets nth child of el, ignoring hidden children, sortable's elements (does not ignore clone if it's visible)
and non-draggable elements
@param  {HTMLElement} el       The parent element
@param  {Number} childNum      The index of the child
@param  {Object} options       Parent Sortable's options
@return {HTMLElement}          The child at index childNum, or null if not found