I wouldn't give them veg. scraps until they are older- maybe three weeks or later. I say this because my chicks are never interested in treats until four weeks or so. Other people on here give treats very early.
However, I do get my chicks onto grass if possible at three weeks, and they come in and out of the brooder to their heat lamp, back to grass, etc. They are not smart enough to get back to the heat lamp until 3 weeks. Even then you have to watch. All this is weather dependent, of course.
Really, it is very good to give them some kind of greens (grass or veg. greens) by three weeks because I personally believe those fresh vitamins are essential.
McMurray Hatchery chick care page recommends sprinkling chick grit over food like salt at day three (not a grit hopper). This is what I do. Technically they don't need it until they ingest things other than cracked grains/seeds (like grass) but I like to give it anyway as I see them ingesting shavings and other things.
At age two weeks they can have a grit hopper, because that is the age at which I have found that they don't overindulge in the grit (can cause impacted crop I have read).
Sand or cherrystone brand grit #1 is good for up to 4 weeks of age, size #2 is for 4-7 weeks (and I give this when I have bantams), and size #3 is for adults. Size #4 is for turkeys, it says on the bag. They have a website with sizes in inches so you can see if you have the correct size if you are using a gravel pile as grit.
If free ranging they can find grit in the soil if you have sharp gravel in the soil. I give my chickens grit because they eat whole grains, which they cannot grind without grit.