Sox rising stars Will Middlebrooks and Daniel Nava hosted “Red Sox Kid Nation Winterfest” at Fenway Park on Saturday. The afternoon included time in the batting cages, train rides, magic, face painting, and a video game competition. It also included activities for parents — specifically, the chance to audition to be the new Fenway Park public address announcer. The ballpark already hosted private auditions for the position last week, with a panel of judges that included players Jonny Gomes, David Ross, Ryan Kalish, and Nava. Saturday’s walk-up hopefuls for the job included David Delorenzo and Jim Scriven.