Grit is just little rocks. Any brand should be equally good, as long as the rocks (grit) are the right size for the chicks to swallow.
If they have to work that hard to find small pieces, you probably don't need to worry very much. Hunting for each piece will slow them down, so they cannot just gorge themselves on shavings and then decide it was a bad idea.
Once they spill some food in the shavings, some of what they dig up & eat will be food instead of shavings. (I do not suggest that you dump food on purpose for them to find, because chicks always spill plenty on their own.)