As the child of alcoholic parents, I know I knew from a very young age there was something wrong with myparents. I of course didn't know what but I knew that my parents weren't like other parents and I also knew it was my fault somehow.
I would be truthful in an age appropriate way. Making up little white lies (such as daddy isn't feeling well) doesn't help IMO. Kids may not have the words to say dad is drunk but they know what not feeling well looks and feels like and they know that is not what is going on with dad. Don't assume that dad's drinking is under their radar either. I knew darned well when my parents were drinking (without being able to say that what it was) and I learned very young to stay away from them when they were drunk.
Giving them accurate age appropriate information will empower them, will help them understand on a gut level that this is not their fault, will help them learn their own healthy coping mechanisms.