Child Care Benefit is paid on a means tested sliding scale basis to everyone (but means tested).
You dont currently have to do anything to qualify for the minimum 24 hrs CCB - but your eligibility for a benefit % is means tested.
As b_r said, Child Care Rebate is for people working or studying more than 15 hours a week up to an arbitrarily elected $$$ number each financial year (currently $7500).
You can elect to receive this quarterly or annually, however as b_r said, it is changing in the 2012 financial year and families can elect to receive it or have it paid to the service fortnightly.
For both approved and registered care, both parents must be working or studying to increase 24 hours to a maximum of 50.

Only 'APPROVED' care is eligible for CCR.
As far as I know APPROVED care is only different to REGISTERED care, in that approved carers participate in government stipulated quality assurance and meet the requirements for QA.
For example NCAC (national child care accreditation council) or NQF (national quality framework), are the 'regulating' QA bodies. If a centre of FDC were to fail accreditation they may no longer be considered an approved type of care.

As far as I can tell a Nanny would not make you eligible for CCR.

HTH.