I don't know how big they are when they are born, but when you see the babies you should catch them. What I used to do was put them in a gallon ziploc bag and keep the bag in the tank. Then put the bag so that the lid of the tank is held down by the tank lid. Feed the fry flake food and do a water change in the bag by pouring out some of the water into the tank and refilling the bag. After about a week or 10 days they should be big enough to put back into the tank and not get eaten.
HTH
Scott