Can I help out for a few days without registration?

Yes, if your contact is incidental rather than regular: no more than 3 days in any 4-week period and no more than 7 days in any 12-month period. You need registration if you have regular, more than incidental, contact with vulnerable people in a regulated activity and no exemption applies.

Source: Access Canberra, Check if you need WWVP registration before applying, and WWVP scheme.