I only give hard boiled egg yolk to the young ones. I put sand in it also. The older girls(11 weeks) will get a piece of homemade wheat bread, their favorite and fruit once in a while. I don't give treats often, just because my chicks seem really skinny to me.
Try chopped up boiled egg mixed with cooked spaghetti and some finely cut greens (grass and clover are fine). Mine have been eating this since they were days old and love it. The spaghetti add the worm factor
They are 5 weeks now and like well thawed frozen corn, tomatoes, strawberries and pretty much anything of the fruit or veggie mixed in as well but the basic egg/clover/spaghetti is still their absolute favorite
Someone here posted about putting greens in a suet cage and letting them peck at it. This has been far more successful for my 3.5 week old chicks than when I put the greens loose in the brooder. Not sure why....
Mine love quick oats. It's cut into smaller pieces than regular oatmeal, so easier for them to eat. They also love clover, grass, and this purple flower I found (looks like phlox, but was growing wild on a taller plant). My 10 week olds love grapes, maybe the younger ones would like them if they were cut up small enough. My 4 week olds don't like them, yet. I still need to try cooked spaghetti, I'll have to cook some tomorrow.