Hello there and welcome to BYC!
Most chickens know not to eat poisonous things and generally only do so if they are starving. However one never knows with a chicken and sometimes they do eat things they shouldn't.
If they didn't eat much of it and weren't on the lawn too long, it may not be a problem. If they are going to get sick, you would probably notice sick birds in the next day or two.
Here is a thread on "flushes" you can use to remove toxins from their bodies. Use one of these for the recommended time and these should help flush poisons out.
If you do have an avian vet around your area, you might call them as well and see what they think about this.
Good luck and I hope they are ok!