If you are still not convinced that you want to keep your dandelions and want to kill them, here is a non-toxic alternative to try - Vinegar.
http://zduckman.hubpages.com/hub/Non-Toxic-Weed-Killer
Better yet, don't use anything on your lawn and just pull by hand. All parts of dandelion are edible - even the roots. The young greens are great in salads. We rely on weed killers way to much in our country. Even if your chickens get a trace of herbicide which is probably labeled as non-toxic...