Knox and Vivienne Jolie-Pitt's older sisters and brothers love to lend a hand: "[They] pick out their clothes and help change and dress them," proud parents Angelina Jolie and Brad Pitt told PEOPLE last year. So to celebrate the twins' first birthday July 12, Maddox, 7, Pax, 5, Zahara, 4, and Shiloh, 3, all helped bake two cakes: one for Knox, one for Viv. Grammy and Papa (Pitt's parents, Bill and Jane) flew to L.A. from Springfield, Mo., for the family party. Born last summer in Nice, France, "Knox and Viv are doing great," says a close family pal. "They're happy and healthy." And officially toddlers! "They've both taken their first steps," the pal adds—and they're keeping Mom, Dad and all their siblings on their toes chasing after them.