

Did you get pictures of the dog with the chicken? I imagine it would be difficult to remember a detail like that when you're in such a situation, but whatever evidence you can collect will help. Can you approach the dog to tie it up in your yard?
Find out the leash laws in your community, in many places it is illegal to let your dogs roam unleased, even if walking alongside their owner. If that is true in your area, call AC every single time you see these dogs running loose. Try to enlist other folks in your area. A dog that size is a real threat to any child, even to adults. Even a friendly dog can do real harm just trying to be friendly. Put the AC number on your speed dial. Contact your local commissioner. Call the local news stations.
Okay, I'm out of ideas. But in addition, you will have to amp up the security in your coops, which must be difficult to try & keep out a Great Dane. Do they jump your fence, paw their way into the run? Maybe some electric wire around the pens would help.
Here's another
