Poppycock. Joffrey being dead is Joffrey's fault, although perhaps you could say Cersei was a bad mother who raised a demon. But Joffrey died because he was a miserable shit with power. Tommen... that one's mostly on Cersei, either by letting the Faith Militant come into power and brainwash him in the first place, or by murdering everyone and causing him such grief he felt suicide was the only option. Myrcella... I don't see how you possibly hold Cersei accountable for Myrcella's death. Cersei didn't want her to go to Dorne in the first place, and it turns out she was right to be paranoid about that. Cersei deserves blame for a lot of bad actions on the show, but I wouldn't hold her accountable for her children's deaths.
Cersei had multiple chances to reign in Joffrey and never did, she only started to try after it was too late. And Tommen is 100% Cersei's fault because she let the Sept rise into power and then she also murdered Tommen's wife.