The younger the birds the fewer treats you want to give them. They should get most of their calories and nutrition from their starter.
I give green feed to all of my birds, chicks included. Starting at one week I'll finely chop some plain old cabbage, put it on a jar lid, and let them have it. They'll be suspicious at first (usually) and won't eat much, but they'll eventually realize that it's 'good stuff' and eat it more heartily. Don't feed more than they can clean up in about fifteen minutes or so. After a week or so you can begin to slowly chop it more coarsely until about about five weeks or so I just put a big chunk in whole. They'll have figured out by then how to tear off bite sized pieces.
Every other day I'll sprinkle rolled oats (like from the grocery) over their bedding to encourage them to keep it turned up and fluffy.
When you start with the treats be sure to start them on grit as well. Clean, coarse sand will work or you can get canary grit from the feed store (not the high calcium kind). Just sprinkle a little over their feed like you were salting it.
I don't give anything other than that except for whatever flying bugs the brooder light attracts. I brood chicks outside so in the warm parts of the year that can sometimes be considerable.
.....Alan.