With shipped chicks, I usually wait until they are a week old, just to make sure they are stabilized and not having any problems. Then I grind or chop things into tiny bits for them, in addition to giving sand or very fine grit.
Since they are growing so fast, I try to give them things that are really good for them. Things that will build a healthier chicken, rather than just something they'll like. I also make sure I'm not unbalancing their diet by feeding a lot of just carbs, for example.
What kind of treats are you thinking about feeding them?
I don't feed my chicks any treats at all, other than scrambled eggs. They do not need grit for that. Commercial feeds are made with the vitamins and minerals they need, and it really is best to stick with at least 95% of their diet being that.
Mine are just 5 weeks and the only thing I'm wiling to add at this age is animal protein, because, gosh darn it, the feed has only plant based ingredients. A little meat protein goes a loooooonnnnnnnggg way!