function selectAll(obj){
    $('input[name="xxx[]"]').attr("checked",obj.checked);
}

<input type="checkbox" id="mother" name="mother" onclick="selectAll(this);"/>全选

<input type="checkbox" id="son1" name="xxx[]" />子框<input type="checkbox" id="son2" name="xxx[]" />子框<input type="checkbox" id="son3" name="xxx[]" />子框<input type="checkbox" id="son4" name="xxx[]" />子框

function selectAll(obj){
   if(obj.checked) {
    $('input[name="xxx[]"]').attr("checked", true);
   } else {
    $('input[name="xxx[]"]').removeAttr("checked");
   }
}

function selectAll(obj){
 var xxx = document.getElementsByName("xxx[]");
  if(obj.checked) {
   for(var i = 0;i < xxx.length;i++) {
    xxx[i].checked = true;
   }
  } else {
   for(var i = 0;i < xxx.length;i++) {
    xxx[i].checked = false;
   }
  }
}