You can obviously get from the shop the baby food, like tetra or hikari baby powdered food, liquifry,baby brine shrimp, etc.. Finely crushed flake would do, but you are better of with special baby food which has more protein. I used to boil an egg for mine, then mix tiny bits of the yolk with water until the yold dissolves well and then drop super tiny bits where the babies are. All guppies love that but be careful how much you put as it makes the water dirty. The babies dive into it once they figure out its food .
You know that if you don't separate the babies, most of them will get eaten by the adults if there isn't enough hiding spots in the tank. A breeder net, or plastic breeder box that you hang inside the tank will do until they grow big enough not to get eaten.