如果父母是公屋屋主，而子女又想買二手居屋，需要咩申請手續和資格?

Nothing is related based on what you have mentioned.

To purchase a HOS property, the first question is what kind of HOS property you want to purchase - the one with or without sale restriction (market difference paid or not).

For the one without the restriction, nothing further is needed except the money for the property.

For the one with the restriction, the buyer then must qualify under either Green or White Form.

Your parents matter only if the public housing unit will be used for eligibility purpose (Green Form).