Technically your chicks should have size #2 grit now (according to the bag of the below link sold by my feed store):
http://cherrystonegrit.com/grit_sizes.html
And size #3 after about 7 weeks, which is in a couple of weeks for you (so why buy size #2 unless you have bantams, eh?)
Chick grit is size #1.
Sand works for tiny chicks but yours are kind of old.
If you are feeding a commercial ration with no treats or grass then they don't need grit. If you are free ranging they can find their own grit. If you are keeping them confined but they have access to the soil and you see some sharp pebbles in there, you are fine, IMO.
If you just have clay for soil and they are confined, if it were me, I'd get them some grit. If you can just find a small bag of chick grit for sale at the feed store that is what I would do, and feed them the larger grit for adults after around 7 weeks.
Here is a nice page:
http://www.shagbarkbantams.com/faqgrt.htm
Since your chicks are older, you can put a bowl of grit in the run for them (or the coop). Tiny chicks will fill up on grit and it can cause impacted crop since they think it is food. If you notice them eating it too fast you can sprinkle it on the food "like salt."