"I am ashamed of you."
Translation:Wstyd mi za was.
"You (my children) did something awful and I am ashamed because of that and because I am your father and I failed in your upbringing" - I'm not sure if that's the only interpretation that English allows, but it's the one in Polish.
"za was"/"za ciebie"... well, I don't know how to explain it, but that's just what Polish uses in such context. I guess it's also similar in "Zrób to za mnie" (Do it instead of me, while I'm gonna lie on the sofa and watch you work), and probably some other constructions that do not come to my mind now.
"Wstyd" is the noun form of "shame", I guess "wstyd mi" here is more of a fixed phrase. Using a verb: "Wstydzę się" would work perfectly as well (no 'mi', then - it's like 'shame to me' vs normal 'I'm ashamed').
Hmmm... I'm not sure if it's the best way of treating it... maybe? Depends on the specific sentence...