Next Post in Category help

Hello,

For my website I need to have Next and Previous episode buttons. All posts(episodes) have one Parent category named Anime and one child category with the name of the series.

I’m using the following code for my next and previous buttons:
<?php previous_post_link('%link', 'Previous Episode', TRUE); ?>

The problem is, it shows the next and previous posts for the head Anime category, and not for the individual series. How can I get it so it only shows the next post in the child category?