The child star is of a slightly different kind than the actor who happens to be young. Though there have recently been many terrific performances by young actors in young parts, with Quvenzhané Wallis in “Beasts of the Southern Wild,” Jacob Tremblay in “Room,” and Brooklynn Prince in “The Florida Project,” not to mention the “Harry Potter” kids, the niche culture we live in now produces child stars who are designed, almost for the first time, to appeal pretty much exclusively to children their own age. Where Shirley Temple and the “Our Gang” stars made entertainments “for the whole family,” their precocity aimed upward at pleasing grownups, the trajectory today runs laterally, with the precocity fired horizontally at the stars’ own generation. The result may be shorter performing careers, but sometimes also a second act on the best-seller lists, when the audience that once watched them on TV ages into buying books.journey-inline-newsletterinline-newsletter