My daughter got caught flipping off another child?

My daughter Bethany got caught flipping off another child during her kindergarten class today. My EX wife Stacy is pissed at me she said she got that from me. I feel horrible she did that and i feel bad for the kid i shouldn't being doing that in front of her. I came over to tell her that is wrong to do but Stacy and i just got in an argument as usual. I've tried to ignore her and not fight i know it's not healthy for Bethany but she just knows how to get under my skin. What should i do?