Query not returning child posts for any parent post ID

It seems the problem is that for parent/child relationships, I’m using the Toolset Types plugin, which stores the relationships as fields rather than how WordPress normally thinks of them. This means that when WordPress looks at a post ID, it doesn’t see the child relationships that the plugin does.

To get around this, I used this instead of the ‘parent_post’ parameter:

'meta_query' => array(array('key' => '_wpcf_belongs_artist_id', 'value' => $parent_artist_id ) )

This searches in the field that the Types plugin uses to find the post ID.