Gabrielle Grady died on Valentine’s Day last year, three days after her father Christopher Grady plunged his car into the River Avon in Evesham, Worcestershire, as she sat on the front passenger seat.
Her brother Ryan, who was six at the time, was on the back seat of the silver Vauxhall Vectra estate as it began to sink but survived after being pulled from the freezing water by police.
Grady, 42, was convicted of murder and attempted murder by a jury at Birmingham Crown Court.
The children’s mother, Kim Smith, 37, punched the air as she left the court building and said: “I am happy, don’t get me wrong, very, very happy.
“Nothing is going to bring back my Gabby but at least I know that justice is going to be done, and it has been done.”
The judge, Mr Justice Lindblom, told Grady he would face a mandatory life sentence when the minimum term for the offences is decided next Tuesday.
Speaking on the court steps, Miss Smith told reporters: “Although Christopher Grady is facing a deserved jail term, this does not take away the fact that myself and my family were robbed of our Gabby just over one year ago and that will stay.
“We are still living with the loss and devastation caused by his actions on that day.”
The three-week trial heard Grady had warned Miss Smith she had 10 seconds to say goodbye to Ryan and Gabrielle before he drove into the water at Hampton Ferry on the morning of February 11 last year.
The jury of seven women and five men heard Grady drove into the river after a series of arguments with Miss Smith, his ex-partner, over his access to their two children.
Miss Smith said he drove to her house in Abbot’s Walk, Evesham, at around 9.15am on February 11, telling her to say goodbye before driving away shouting the word “river”. She said his face was “contorted” and “vile” with anger.
Miss Smith, who shook visibly as the guilty verdicts were returned, described the moments outside her home on that morning as: “Horrifying, the worst experience ever, (I was) absolutely shook to the core.”
Grady and Ryan were pulled from the water by police and firefighters at the scene but Gabrielle was stuck in her father’s submerged car for two hours. The court heard that her body temperature was just 14C when she arrived at Birmingham Children’s Hospital after being pulled through the vehicle’s sunroof by police divers. She was pronounced dead three days after the incident.
Miss Smith added: “What must also not be forgotten is what my son Ryan also went through on that day.
“Not only the trauma of the events but that he also lost his sister who was also his best friend.
“I am extremely proud of him as I am my other two children, for their strength despite their young ages.”
She said Ryan was doing well despite his ordeal and had asked her this morning about the outcome of his father’s trial.
She said: “He’s seven now and he’s doing really well, really well. He’s not got anything wrong at the moment but we keep things in mind, he’s a little boy.”
The judge told the court that he had yet to read victims’ impact statements in the case and wanted to know more about the effect of the offences on Ryan.
He said: “In the course of the trial, the court has not heard very much, if anything, as to the effects of what took place on the 11th of February 2010 on Ryan, in a psychological sense, either in the short term or the long term.”
In a short written statement to the media, the mother of Christopher Grady, Valerie Hubbard, said: “Unfortunately there are no winners in this case. We are all losers, as our darling Gabby is no longer with us.”
Grady, of no fixed address, showed no emotion as the jury’s verdicts were returned after five hours of deliberation. He will be sentenced at Birmingham Crown Court on Tuesday morning.