Update matrix onBeforeSaveUser
I did the trick so here is the code if it can help someone else

craft()-&gt;on('users.onBeforeSaveUser', function(Event $event) {

   $matrixData = array();
   $matrix     = $event-&gt;params['user']-&gt;getContent()-&gt;children;

   foreach ($matrix as $block){

      $block-&gt;getContent()-&gt;childrenDateOfBirth = '2000-10-10';
      $matrixData[$block-&gt;id] = $block;                                     

   }

   $event-&gt;params['user']-&gt;getContent()-&gt;children = $matrixData;

}