Yes, tell your kids that you smoke and drink because if you don’t, when they see you drinking and smoking, they will ask and the conversation will be different then.
Take the conversation one step at a time, do not bombard them with so much information at once, go slowly and be thoughtful when you are talking to your kids about it. If your kids are old enough to known about drinking and smoking, then they might be shocked to know that their parent is involved in such activities.
You need to have a solid explanation for your actions. Let them know the benefits and the disadvantages that come with your action.
Is will make it easy for them to ask your questions when they are unclear on certain things related to drinking and smoking.