I do offer treats the 1st week

maybe a little egg (1/2 boiled egg
yolk) ,
1 grape finely chopped (this is for the whole brooder), you get the picture,
very small amounts and given in the afternoon when they have been eating their feed all day.
I increase the amount over time, currently I have 6chicks that are around 7 1/2wks old, they share
1 hard boiled egg once a week as a treat, that egg is gone in about 2minutes. They only get 1 "treat" a day, it could be 1 kale leaf, 1 medium large broccoli stalk, 1/4 large apple, etc. Moderation is key with growing chicks. As they grow, the treat size grows slightly, but I still try, even with adults to limit the daily extras.
I've found if you make a big deal out of something, chicks think just about anything you bring them is a treat. I usually hand feed a little of their wet mash to them when I come to re-fill it, they are enthusiastic. Even my adults think that it's the most wonderful thing to get hand fed out of the mash container