SURNAME

hi mommies! I just want to ask for your opinions. I am currently 6 months pregnant, my eldest is already 9 years old. Si eldest is using my surname, kasi not around si lip when i gave birth to him. and now im pregnant again and lip is around po, im confused lang whether my baby should also use my surname para same sila ni eldest ko or iallow ko na na surname na ni lip si baby? and any ideas po if pano ang process if papalitan ng surname yung eldest namin and kung gano katagal ang process? would appreciate your inputs mga moms.. thank you ?