They will need grit of some sort to process the grass. If they haven't eaten too much, you might be able to head off any problems by giving them some as soon as possible. If they were outside eating the grass, they may have picked up some small stones for grit at the same time. If they filled up on the clippings, without any grit, you may have some impacted crop issues to deal with. Hard to remedy in young chicks. You'll probably just have to wait it out and see if they get through it OK. Maybe someone else om here can offer a better suggestion. Good luck to you.