code: function _getChildren(tile) {
    if (tile[2] > z) throw new Error('input tile zoom > ' + z);
    if (tile[2] === z) return [tile];
  
    var children = tilebelt.getChildren(tile);
    return _getChildren(children[0])
           .concat(_getChildren(children[1]))
           .concat(_getChildren(children[2]))
           .concat(_getChildren(children[3]));
  }
docs: [private function]