Is there any reason this conditional wouldn't work?
{standard_page_events var_prefix="cat"}
{cat:event_category_description}
{exp:playa:coparents var_prefix="event" channel="events" child_id="{cat:entry_id}"}
{if {event:count} == 1}<div class="banded"></div><hr>{/if}
{/exp:playa:coparents}
{/standard_page_events}
{event:if count == 1}
.