When Erin Tally took her two-year-old son Aidan home from Children’s Hospital on the day after his urinary surgery, she also brought a new friend: a 4-foot-six-inch, 17-pound, two-wheeled robot who would help deliver care for her recovering child. The robot, made by Vgo Communications Inc. of Nashua, eliminated the need for Tally to drive Aidan into Boston every three days for check-ups. It’s part of a pilot program at Children’s Hospital.