Brad Pittis devastated by his children’s decision to distance themselves from him. According to insiders, the Once Upon a Time in Hollywood actor, 60, has been shocked and saddened by his kids Vivienne, 15, Shiloh, 18, and Zahara, 19, ditching his last name as he continues to battle things out with his ex-wife Angelina Jolie. “Given the cold climate between him and Shiloh’s mother, he suspects Angelina had a hand in this,” the source claimed about the teen’s recent legal filing to officially cut “Pitt” from her surname. “The days after Shiloh’s birth in Namibia were some of the happiest days i…