Hey there, I have been giving mine treats from 3 weeks, small bits of lettuce, occasionally one of our stone baked pizza crusts with nothing on them to keep their little minds occupied. Once or twice a green onion stem, mealworms (once they figured out what they were they LOVED them) bits of broccoli, apple and other fruit, cooked quinoa and recently a cooked edamame bean or two (I have four Buff Orps, so a few edamame go a long way LOL) Yesterday we tried some plain yogurt, they loved that (6 weeks now). Very small amounts, they now know me and the children as the giver of all bribes and will sit happily on a leg or a hand or even a shoulder in the yard. I haven't tried any corn or oats yet, I'll wait till the weather cools off since they have plenty to scratch for in the yard now as spring is warming up but I've seen some people give cooked oatmeal too. Today they got a few tiny bites of a non-sugared cake I had made with apples.
They look excellent and have never had runny poo or any distress.