Drush command to switch to a multi-site child directory?
drush status produces a Site Path in its output which is what you're looking for. With some additional shell commands, you can parse this out.

MYPATH=/srv/www/drupal
MYDOMAIN=example.com

SITE_PATH=$(drush status --root=$MYPATH --uri=$MYDOMAIN | grep 'Site path' | cut -d':' -f2 | xargs)
cd "$MYPATH/$SITE_PATH"