We use a medicated chick feed for a Starter Feed, add probiotic with electrolytes to their water, and also give our chicks live mealworms (up to about 3/4" in length) from their 2nd day. They LOVE the mealworms!
Why mealworms and why so soon? It's what mama teaches them to eat at an early age. We watch our mama hens and how they teach the babies, what they teach the babies, and we've tried to mirror that to some degree. They feed the 2 day olds worms, so we do too.
BTW, if you've never seen ten (10) 5-day old chicks go NUTS over live mealworms being dropped into their brooder during the day, you're missing out on one of the funniest things you'll ever see your chickens do!
They race to the worms, grab one or two, run away ducking and diving, sometimes spinning in circles, and generally playing keep-away to keep others from stealing their worm(s) from them, then try to find a safe space to get it down. Other times, one will get one in her mouth and another will get the other end, and a tug-of-war ensues until either the worm breaks or one chick wins out. IT'S HILARIOUS, and THE CHICKS LOVE THE WORMS!
Try it sometime. Your chicks will LOVE you for the worms.........