Wearing a “I Love Pandas” t-shirt and clutching a panda-covered diary, Kelsey Lambert bubbled with excitement as she glimpsed the real thing. She and her mother, Alison, had made a special trip from San Antonio, Texas, just to watch the National Zoo’s furry rock stars casually munching bamboo and rolling around on the grass. “It felt completely amazing,” Kelsey, age 10, said Friday. “My mom has always promised she would take me one day. So we had to do it now that they’re going away.” Mei Xiang, Tian Tian, and their youngster Xiao Qi Ji, three giant pandas from the...