Justice Graham Lang said the 13-year-old would stand trial in November, and the Youth Court was told today it was intended the 12-year-old would also face trial then.
The 13-year-old will stay in custody until his next appearance in August. Before then a forensic report will be undertaken to assess the boy's fitness to plead.
"I'm not satisfied that he will be able to understand the seriousness of the charges and the court needs to be satisfied of that," his lawyer Maria Pecotic told the High Court.
Justice Lang decided the 13-year-old's name suppression would continue for fair trial reasons and because it could result in extreme hardship for the boy. The judge said suppression would likely continue until the trial.
Members of the 13-year-old's family arrived in court just as he appeared and yelled support for him as he was taken into custody: "Love you bro" and "be strong baby, I love you," they said.
Mr Kumar, 57, was stabbed in the neck in front of his wife about 7.30am on June 10.