Like Pandora said if you can't find chick grit parakeet grit will do. You want it to be the plain parakeet grit, not the kind fortified with extra calcium.
When my chicks were little and before they were on the ground much, I just went out to our gravel driveway and scooped up a pan of dirt...
If they are eating foods other than their chick starter and they don't have access to dirt to find their own grit, then yes they need to have it offered.