From my experience chicks just aren't all that interested in treats until they get a bit older. However, you can begin giving them treats now. However, they will need some chick grit or sand while they are still little.
When they are older, they can find grit in the soil while free ranging, or you can buy grit size #3 cherrystone at the feed store.
I like to give grass clippings that are about 1 inch long for smaller chicks and when they get older 1-2 inches in length.
They will keep needing the high protein of their feed so don't give too many treats while they are growing.
My chicks are about 2 1/2 weeks old, and have been getting soft treats for a few days now. They've had cooked carrot, peas, uncooked strawberry and blackberry, and a few crumbs of pumpkin bread.
As someone else said, just make sure they get some grit if you'll give them something to eat other than chick starter. I read the good advice to offer grit well in advance of offering treats (a day or so) so that all the chicks get some before they get any food in them.
My 3 week old chicks LOVE dry oatmeal and oranges, they won't touch dried mealworms though. Today I gave them a half ear of fresh corn and they really like that too, I've also given them bird seed. Here's a pic of three of them I took last night, it's a cut orange with oatmeal sprinkled on top they went bonkers for it