There is no need to fill this in if you don't have a current DBS certificate.
GSAL are asking that volunteers have enhanced DBS certificates allowing them to work with children.
Please answer for each parent who has a DBS certificate and might come to GSAL.
If you do have DBS you wouldn't be asked to do extra volunteering, you'd just sometimes be asked to do a task for which DBS is essential e.g. group leader or group assistant.