Child theme files

Found the "Checkout our growing community network, why not join one of them?"
in random-groups.php

. ..I 've made a change in the text in the parent to see that it shows up on the site...and it does.
My question is... when I copy this file to the BlogsMu Child folder...I can't get the changes to show.
Can you help?

I would do well to say that the child theme is enabled in the site admin

My next question then is... how do I create a new site (subdirectory) using the child theme and not the parent?

Thanks in advance