At this point,, not much you can do. I would avoid getting any more pulled, and offered to chickens. I do suggest you get rid of it if you can.
There are many different varieties of spurge. My reading revealed that goats and sheep can eat certain varieties. People should avoid it, and are susceptible to complications, but article did not specify Fatality, Suggested a trip to Emergency was possible.
Your chickens most likely just ate some because they assumed it was a treat you were offering.
Most chickens do know what to not eat. I have Lillie's, and other toxic/poisonous plants/flowers growing in my yard. My chickens do not eat them. Have been keeping chickens for over 20+ years now, and had no deaths due to eating poisons.
I only keep chickens as pets, and they all live into their Golden Years, My current oldest now is 9 years old.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,