Its easier if you just stick this at the end of your page controller init function rather than use the template

public function init() {
parent::init();

        if($this->RedirectToChildOne && $this->Children()->Count()){
        Director::redirect($this->Children()->First()->AbsoluteLink());

This feature is invaluable if one is using jquerymobile since otherwise the root page is invisible and its content has to be moved to be its own first child

Hi, just in case anyone wants a variation...I just test to see if the page has any content, then redirect to the first child if not.

if(!$this->dbObject('Content')->hasValue()){ 
    if($this->Children()->Count()){ 
    Director::redirect($this->Children()->First()->AbsoluteLink()); 
    } 
}

As Director::redirect() is deprecated, this is what i used:

public function init() {
   parent::init();
      if($this->Children()->Count()){
      return $this->redirect($this->Children()->First()->Link());
   }
}

Code i added works but it affects also admin so at the end i solved this issue in template so that's what i added to my menu code in Navigation.ss:

<% if $Children %>
<a href="$Children.First.Link" title="$Title.XML">$MenuTitle.XML</a>
<% else %>
<a href="$Link" title="$Title.XML">$MenuTitle.XML</a>
<% end_if %>