What a scumbag. Driving drunk with your children and then putting them in the middle of a conspiracy not to tell you. Thank goodness they weren't injured by his brainless, selfish behavior.
You are going to have to step up and protect them. Make sure they know that their dad is an adult who is responsible for his choices and that they did the right thing by telling you. Make sure they know NEVER to get into a vehicle with a driver who has been drinking. Give them a way to contact you or someone they trust for a safe ride home.
This just makes me see red. So sorry your children experienced this. Now, what are YOU going to do to protect them in the future?