If my son and I have been subpoenaed by the defense as witnesses in a domestic violence case what happens if we do not appear in court?

You indicate it is the defense which wishes you to appear and not the state. It would be rare for the state to seek to hold a witness in contempt on failing to appear, and even more rare for the defense to do so. Further, if someone mailed something which you did not receive, which is different than being presented an item and refusing to sign for that item, there is an argument you have not been served at all for the new hearing. There is also the question of why the defense is calling you. A person's liberty is at stake. If you have truthful information that weighs in favor of the defense, not appearing provides an unfair advantage to the government, which already has the advantage of greater resources and greater reputation and greater leverage. I leave you with the thought of there is that which one must do, and that which one should do. Please do that which should be done, even if you feel you are not forced to appear.