Just to clear up possible confusion around birth certificates.
I had a debate about this with my initial lawyer before I ditched them 
The birth certificates are only required to prove the relation between the main applicant and their dependants, as follows:
- for children, the child’s birth certificate required;
- for parents, the main GV applicant’s birth certificate;
- for in-laws, the spouse’s birth certificate.
There could be other less common scenarios e.g. adopted children but the logic remains the same.