There are basically 2 schools of thought: Medicate them to kill any bacteria they might pick up in their world...or feed them a healthy diet so their immune systems are strong and they can cope with any bacteria they might encounter. I choose the latter!
I have had really good success for baby chicks from birth using simply small whole grains (quinoa, flax, millet, amaranth--which I get at a health food store) plus larger ground-up whole grains/seeds (brown rice, wheat, barley, sunflower--I use a little cheapy hand-grinder, designed for coffee beans--to crack the grains up). I only use this for the 1st 10-14 days, then start graduating them to non-ground whole grains...which is what I feed my older birds.
I also give babies sprouts, chopped up spinach, grass, lettuce, tomato, peas, apple, banana...a little yoghurt, butter, minced garlic sometimes...and I like to also add to the butter or yoghurt a powdered wheat grass (from health food store) for added greens, phytonutrients. I do not add synthetic vitamins...
Remember to include grit, of course, to help them "grind" their own grains...I use both parakeet grit (almost like sand) and Hi-Cal grit (for cocketiels?) from
PetSmart until they get to be a couple months old.
I start chicks on outdoor ranging as soon as possible, for a few hours to begin with, within a few days of hatching if the weather is decent. I also don't give them tap water...or if I have to for some reason, I let it sit overnight or boil it first to get rid of the chlorine.