As per the article here there is a widespread misconception that if puppies don't get mother's milk for complete period, then they won't build a good immune system. In reality, they get all the antibodies in first 24 hours of nursing. Also, if the mother has not faced and fought off a particular disease or is not vaccinated.
We are feeding them Cerelac every 4 hours for now. We were told by Red paws rescue that a better option is Royal canin babydog milk but we make do for now.