Thank you very much for all the responses.
Conclusion:
1) Change the birth certificate to reflect the biological mother
2) After acceptance of Philippine Civil Registry, get a PSA certified-BC
3) Inform US Embassy Manila of Additional K2 Beneficiary
4) Apply for K2 Visa as "Follow To Join" next year.
Note:
1) During the medical, Fiancee declared 3 biological children.
2) Fiancee will tell consul during interview of the 3 children. Only 2 were listed on I-129F due to uncertainty on the first child, birth-registered as a sibling. Per conversation with previous embassy staff, this apparently is a normal situation for many single mom K1 Beneficiary in the Philippines.
Recommendation:
For future filers, declare all biological children of a K1 Beneficiary and correct the birth certificate as soon as possible.
Best Regards,
PeterM