At about 2 weeks of age, I put quartz "playbox sand" in the bottom of the brooder with some pine shavings on top. The chicks dig down & eat some and this serves as my chicks first grit.
The grit you buy for chickens is "granite grit." Granite is an igneous rock with the primary mineral (and the hardest mineral & what defines it as a granite) being "quartz." Quartz is what gives granite its resistant properties and makes it a good grit for chickens.
Qurtz sand is small enough to serve as grit for chicks.
Just so you know, playsand can have some pretty nasty stuff in it like asbestiform tremolite and/or crystalline silica. This stuff is carcinogenic, and can be bad if inhaled, so I'm guessing it's not great for little chicks. If you're concerned (which I know not everyone will be), just read the label and make sure this stuff isn't in it.
The owner at our supply store suggested I had super fine grit to my starter feed to help w/ our baby chicks pasting up. I've been adding it & haven't had any more problems. They range from 1 - 3 wks old.