I know some people do give treats to younger chicks but I always wait until mine are closer to eight weeks old before giving any other food to mine. When you do start to give treats make sure they have grit available; unless it's yogurt or such.
Just passing on information from a long time chicken owner-(not me) who told me you can and should give small cut up pieces of greens to even day old chicks.
She told me to get nice new grass and cut it up very small for my chicks every day - they thrived on it! I never had plugged up chick butts either, I think the enzymes in the fresh greens helped their systems out in that respect.
Maybe just grass - not a variety of greens at first or stick to one kind of green?
I have been giving my chicks grit since day 6 and then day 8 I started giving them small amounts of the buds from a head of broccoli (bunny got the rest of the broccoli). No pasty butt issues. I give them plain yogurt, mushed up hard boiled egg, and small bug treats too. Now that they are 3 weeks old I can give them larger pieces of the broccoli and if I dice up the stems really fine they will happily eat them too. I actually gave one group of my chicks a whole broccoli head today and watched while they pulled all the buds off of it with several chicks pulling in different directions. Half of my chicks will fight over who gets to perch on my arm, even when they know I don't have treats, the treats just help with the bonding process but I don't like to over do it either. My chicks will get one treat time per day, not always the same time every day. Sometimes I will skip a day and just play with them on the off day, letting them climb on me and test their little wings flying across the top of the brooder and sometimes over to the other brooder or the couch. One missed the edge of the brooder today and landed right on my cat's head. Scared the heck out of her, the chick was unfazed and just hopped down and scratched at the floor while watching my cat make a hasty retreat
My theory on treats is that if these were chicks hatched naturally by mom she would have them out picking up all kinds of forage by the time they were just a few days old. I have not given them any kind of fruit though as per the advice of some experienced BYCers and my own misgivings about giving baby animals fruits. When they get around 8-12 weeks I will probably introduce them slowly to small offerings of fruits.
Perhaps, like weeder geese, you could train them to eat the weeds in your yard by feeding chopped up weed bits to them from an early age so that they will develop a taste for them. Of course you should look them up first to make sure that they arent poisonous.