I fed my reds flakes, mosquito larvae and bloodworms (those frozen cubes) and very small feeders (like neon tetra's) when they were babies. When they got bigger, I slowly switched to smelt, redfish fillet, shrimps and larger feeders.
I wouldn't feed very small p's "heavy" foods like steak or beef heart: I heard their digestive system is still not fully developed yet, and those foods might be harmful when they are really small (correct me if I'm wrong, though). So I'd go with the stuff I mentioned above (flakes, frozen cubes), and gradually switch to other food sources.
Anyways, good luck!