Hi Ron
Since you are using subgrids to display the relationships, you will not be able to use out of box functionality to force a mandatory response from the user. The easiest way would be to have a javascript or plugin to fire on form save and then check to see
if the relationship is marked.
But my guess is you already have something similar in place to track the response. How are you getting the input or the response of what the user has selected in your custom page?
Thanks and Regards
AniMandal
http://xrmadventures.wordpress.com/