If you're really scared, you can take away their feed for a few hours. Likely, they just finished a big meal. A crop will normally fill and empty as the chicks eat. If they're still running around and looking alert and healthy, they will probably be fine.
I think it's highly unlikely that most of your chicks have impacted crops. I don't think it's a very common problem, unless they've been eating long strands of hay that get wound up inside their crops, or something like that. Like Spring said, they probably just ate a lot and have full crops. Where are they? In a brooder or outside on grass? Are you giving them grit to digest their food (if fed anything other than chick starter)?
unless they got into something they shopuld not have ie: straw, string, plastics, then they are not inpacted just full. if it is the size of a golf ball or gumb ball dont worry they do fill up fast. remove food for a little while and it should go down. if eating only starter feed I would say almost impossible to get impacted crop. but it could happen if they eat a lot of shavings. usually they are not that dumb. Good Luck