Chicks are pretty much programmed to peck at anything edible and inedible. Even if it is "inedible" to us, like small rocks, they need it in their crop to help grind up pieces of edible stuff.
If it's bug-sized and they can swallow it,
chickens will eat it. If it's too big to swallow, they will shake it and break it into smaller, swallowable pieces.
I would suggest putting bread and vegetables through the food processor or blender until it is finely ground up. The chicks will quickly learn what tastes good and what doesn't.
Chickens have much more latitude for "what tastes good" than humans do. The only thing I have tried on chickens that they decided doesn't taste good are ginkgo fruits. And that's probably because the ginkgo fruit is high in butyric acid, the compound that makes rancid butter rancid.
If you grind up things that are good protein sources, peanuts, cooked beans, meat, that
should meet their nutritional needs. If you cook meat until it falls apart, then let it dry out in a frying pan until it turns to
meat floss, that makes excellent high protein chicken feed.