Around here young pullets of a good egg laying breed go for about $20 right now, but depending on what part of the country you are in $25-30 might not be unreasonable, especially if you want one "now". Replacing their chickens with slightly older (better) ones of the same type seems to be fair to both of you. You have apologized and are taking steps to be sure it doesn't happen again, since there does not seem to be any question that your dogs were the ones in the wrong. As for what they could get "legally" for pain and suffering and emotional distress, who knows, people can and will sue for anything, winning is another question, I would love to see how that flies in small claims court. At this point hopefully your neighbors will be decent understanding people and let it go. Knowing them, can you think of anything small you can do as a "good will" peace gesture besides just replacing the hens? Treat for their chickens or something?