On the program cover for the Circa ’21 Dinner Playhouse’s new family musical, right under the names of show creators Marcy Heisler and Zina Goldrich, you’ll find this brief synopsis: “A delightful adaptation of four of Barbara Park’s best-selling books.” The built-in praise seemed a tad presumptuous: Its delightfulness wasn’t (hopefully) going to be our opinion, but was rather a fact? Well ... yeah. With its hugely endearing ensemble and peppy, cheerfully sung songs, Junie B. Jones: The Musical is almost inarguably entertaining, and would likely have been an hour of radiant, capitalized Sunshine even if that weren’t also the first name of its gifted leading performer.