Being a grandfather has put a bounce in my step — frequently followed by a stubbed toe — but I really got a jump on fun when my granddaughters invited me to join them on their new trampoline. I’ve had my ups and downs over the years, but I had never been on a trampoline, even when I was the girls’ age. That’s why Chloe, a third-grader, and Lilly, a kindergartner, were happy to help me reach new heights of giddiness when the three of us cavorted on the springy circular device that was recently erected in their backyard. “You’ll have a blast, Poppie!” Chloe promised. “Don’t fall on your face!” L…