I am wondering the same thing.  I bought a box of bird grit and it is a lot smaller than regular sand.  Grit has fine egg shells for calcium, I think, which we can add to sand if it will work.   I hope regular sand is fine enough grit for them because it's cheap. I think it sounds like a better idea to get a bag of white play sand from a box store; it's probably cleaner. Heck, or maybe its treated with nasty chemicals! That wouldn't be good.  I'm thinking river drag would have nasties in it.  
I am wondering just how resilient chickens and chicks really are.  I have my first 26, 4 day old chicks. If take them to the garden and on a day when it is 85-90 degrees, how does the sun treat them? Do they overheat?  I have noticed that they will be walking along on the warm ground and just fall over asleep. It's  funny.  
I am also curious about what I have been reading about using clay to dust them with. I have some and I would love to hear from someone's personal experience with using kaolin clay or bentonite.