First treats for my chicks - at two weeks old - plain yogurt, chopped hard boiled egg (fun for games of keepaway), millet pods (the kind you give to parakeets) and crickets.
By two and a half weeks old I have them outside eating whatever they happen to find. My mama hen is doing the same with her 12 day old chicks.
Just be sure they have grit of an appropriate size when you start offering treats that need "chewing".