This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
Return to the Sector 6 Slums and go to Aeris' house. Talk to Elmyra, her
mother. After Barret has been reunited with his daughter, return to the
Wall Market. Go up to the top screen where the entrance to Don Corneo's
mansion is, and look for a path on the right side of the screen. You
will see a bunch of kids run along the path. Follow them, and then climb
up the pipe to continue your quest.