Sorry, I did not actually answer the question you asked...
My kids knew a lot wasnt right long before their therapist spoke to them (with me present) about his addiction(s).
My kids are 10 and 7 and my 10 yr old has been anxious, like your 11 yr olds for a long time...
I don't have any magic words or books I found helpful, but a good child's therapist was very helpful to me with helping my kids deal with their dads craziness...