It’s a sign of the times: PBS will introduce a half-hour version of “Sesame Street” in September. Today’s busy preschoolers have no time to waste. For those accustomed to mobile, digital entertainment, an hour-long television program is so last century. Effective Sept. 1, PBS announced, “the additional half-hour program will air on weekday afternoons on PBS stations, complementing the one-hour series that airs weekday mornings (9 a.m. on RMPBS). For the first time, selected full episodes of the 30-minute show will also be available for free online at pbskids.org/video, on the PBS KIDS Video App and on the PBS KIDS Roku channel.” Citing the growing popularity of mobile devices, PBS Children’s Programming boss Lesli Rotenberg said PBS Kids’ rea...