[Skybluebrewer]

If you have not yet applied for the 820/801 and you're waiting to do so until your child has Australian citizenship, then there is nothing else you need to do other than fill out the family details in your partner visa application. You certainly will not need to add the child as a secondary applicant and pay an additional fee on top of the partner visa, if that's what you're asking. The child will not need to be a part of your visa application at all once it has citizenship, with the exception of family information sections.