You can't add the child to a sc. 300 visa that has already been granted. I take it the child was included as a non-migrating dependent ?
A dependent child can be included with sc. 820 application, but the child would need to be in Australia on some other visa without "no further stay" condition.
A dependent child can be added to your sc. 820 application after it is lodged but before the temporary Partner visa (subclass 820) is decided, but the child would need to be in Australia on some other visa without "no further stay" condition.
A dependent child can be added to your application after your temporary Partner visa (subclass 820) is granted but before the permanent Partner visa (subclass 801) is decided. In that case the child must first be granted a Dependent Child visa (subclass 445), which will let them stay in Australia or come to Australia if they are overseas.
A dependent child can be included with sc. 820 application, but the child would need to be in Australia on some other visa without "no further stay" condition.
A dependent child can be added to your sc. 820 application after it is lodged but before the temporary Partner visa (subclass 820) is decided, but the child would need to be in Australia on some other visa without "no further stay" condition.
A dependent child can be added to your application after your temporary Partner visa (subclass 820) is granted but before the permanent Partner visa (subclass 801) is decided. In that case the child must first be granted a Dependent Child visa (subclass 445), which will let them stay in Australia or come to Australia if they are overseas.