I have this function:

    public function LatestArticles() {
        return $this->Children()
            ->filter('PublicationDate:LessThanOrEqual', SS_Datetime::now())
            ->sort('PublicationDate','desc');
    }

It's not outputting anything at all. If I switch it to this:

    public function LatestArticles() {
        return ArticlePage::get()
            ->filter('PublicationDate:LessThanOrEqual', SS_Datetime::now())
            ->sort('PublicationDate','desc');
    }

It outputs of my ArticlePages, but that (obviously) includes all ArticlePages site wide, not just the Children ArticlePages.

If I change the code to this:

    public function LatestArticles() {
        return $this->Children()
            ->sort('PublicationDate','desc');
    }

That outputs all my children ArticlePages but that includes articles that have a publication date later than today.

It seems to me that I'm maybe not using the filter function correctly. Where am I going wrong?

$this->AllChildren()

Children does not show anything that either:
a) a user doesn't have permission for (same for allchildren)
b) has 'ShowInMenus' set to false.

$this->AllChildren()

That doesn't work at all. No articles show up and in fact, it breaks the page. I get a white screen. There is only one user for the back end. That is the default admin account. There are no users for front end. Everything is set to show in menu. Like I said in the original post this code works:

public function LatestArticles() {
        return $this->Children()
            ->sort('PublicationDate','desc');
    }

It's when I try to add a filter that it breaks. Thanks for trying though :(

Oh, sorry I didn't read properly.

So you're either looking to add to the filter when using ArticlePage::get() - 'ParentID' => $this->ID

Or using the string 'now' instead of SS_Datetime::now() - although that may return a string.
And it doesn't explain why AllChildren gives you an error (put your site into dev mode to see it).

It could be an issue with ArrayList and filter modifiers. Not sure though.

With your help I was able to come up with this that worked. Thanks!

    public function LatestArticles() {
        return ArticlePage::get()
            ->filter(array(
                'ParentID' => $this->ID,
                'PublicationDate:LessThanOrEqual' => SS_Datetime::now()
            ))
            ->sort('PublicationDate','desc');
    }