Get a book on etiquette that is geared to their age and reasonable expectations. Read it and tackle one thing a week. Let them know what they need to do. Then demand that they do it. This is not a negotiation anymore than appropriate grade are a negotiation.
It is super important that you and your husband model appropriate behavior. Having said that, sometimes people don't. The fact of the matter is, the right thing to do is ALWAYS the right thing to do, even if everyone they know is not doing it. So, dad's manners are horrible, but that doesn't mean that good manners aren't your expectation.
If they eat like barnyard animals at the table, they get one warning and then their meal is over and they can leave the table. I promise you it will take one time. Oh, there will be anger, wailing and gnashing of teeth. There will be complaining of overwhelming hunger. But, if you stick to your guns and they see that you mean it, it will take one time and they will take the one warning and run with it.
Apply the same method in all areas where manners are expected and they are learning a new one. If they don't say, "please" the answer is "no". If they don't say, "thank you" then take back what you gave them. If they don't say "excuse me" before talking to you, ignore them completely and if they persist send them to their room.
I know it all sounds hard core, but manners at their age should be an easy task to manage for them, so at some point it's a choice and a lack of consideration for others comes into play. Learning how to use manners and etiquette is a huge life skill that will take them rather far and will effect how the world views them. It's good to learn them, now.