Scratch is really a treat, and you don't have to give it to them at all. It's pretty big pieces for young chicks, too, or at least the stuff I buy is. I give them a handful a day (for 11 birds) to get them to turn the litter in the coop. They need starter til around 8 weeks, then grower til they lay, then layer. They don't need grit when young and on chick starter, but you probably need to offer it when you give treats (including bugs.) You should be able to buy chick grit, finer than chicken grit, which is for adults. You could also use sand for grit when they are young.
Grit is granite, a very hard rock. They store it for a while in their gizzard where it is used to grind up their food -- kind of their version of teeth. You may not need to buy it for them if their outdoor area has enough hard stuff for them to grind their food. Our rock is limestone, which is pretty soft, so I provide grit.
You should be able to buy grit at
TSC, if you have one. My feed store doesn't carry it but I'm sure many do -- or talk to them and see if they will order it. I buy it in fairly small bags, maybe a quart, for around $6 or $8.
