There’s a lot of high-speed activity in your average children’s movie — the slapstick-on-steroids physical comedy, the rapid-fire dialog. All of which can be exhausting (see: “Spies in Disguise,” the “Rio” films, “Cars 3”), though it can also be creative, as in the “Toy Story” films or “Sing” or the outsize digital spectacle of “Ralph Breaks the Internet.” The frenetic spirit of “Sonic the Hedgehog” feels formulaic, but it certainly props your eyes open. In that light, it’s unusual to encounter a mainstream kiddie flick as shambling and relaxed and old-school quiet as “The One and Only Ivan.” We are, in fact, so used to seeing children’s entertainment fueled by the ADD impulse that you may think, for a while, that there’s too little going on in the movie, that it’s taking its sweet time for no good reason. But “The One and Only Ivan” pays off in a soulful way. Based on a captivating true story, which Katherine Applegate took off from in her 2012 Newbery Medal-winning novel (the basis of Mike White’s screenplay), it tells the tale of a silverback gorilla, Ivan (voiced by Sam Rockwell), who lives in a partially glassed-in cage at the Big Top Mall and Video Arcade off Exit 8. There, he’s the star attraction in a shopping-mall circus, which features a menagerie of animals and builds to the moment each day when Ivan, rearing up on a platform, beats his chest and roars with fury like King Kong. It’s enough to scare the kids in the audience, but as Ivan confesses to us, speaking in Rockwell’s amiable skewed tones, he’s not really angry; it’s all an act, one he’s been selling for 20 years. What Ivan really loves to do is draw. “The One and Only Ivan” combines live action with digital animals, and the creation of Ivan, in particular, is marvelously effective. He may not have had Andy Serkis to model motion for him, but he’s every bit as expressive a simian as Ceasar from the “Planet of the Apes” films. The difference is that Ivan’s pensive, totally lifelike gorilla face expresses the sadness he can scarcely bring himself to talk about.For a while, “The One and Only Ivan” is a tender creature-feature hangout movie. We get to know Ivan’s fellow critters, who are right out of the Disney playbook: his mangy-mutt best friend, Bob, voiced by a spiky-and-what-else-would-he-be Danny DeVito, plus Henrietta the baseball-playing chicken (Chaka Khan), Snickers the frou-frou poodle (Helen Mirren), and Stella the wise old elephant, voiced by Angelina Jolie, who wraps the most serenely mournful of tones around poetic thoughts like “Don’t you just love the moon, with its untroubled smile?” Keeping all this in check is the circus’s owner and ringleader, Mack (Bryan Cranston), who’s a ham and maybe a bit of a carny-barker fraud, but an okay dude just the same. In flashback, we see that he raised Ivan in his own home (which was enough to chase away his wife). Suddenly, there’s a new arrival: Ruby, an adorable baby elephant who bats her eyelashes like Dumbo and is voiced in sugary tones by Brooklynn Prince (from “The Florida Project”). From the moment that she and Stella wrap their trunks around each other, we know they’re going to have a mother-daughter bond. And as soon as we hear that Stella’s foot is hurting, but there isn’t a thorn in it, we know where that’s all going. (It happens pretty quick.) “The One and Only Ivan” is a Disney animal tale in which not one but two parental critter figures die. But, as in “Bambi,” this device is used not to manipulate the audience but to enrich the stakes of the story.I don’t want to overstate the film’s virtues; it’s no “Bambi” or “Dumbo.” But as written by Mike White, with his gentle knuckleball humanity, and directed by Thea Sherrock, who made the ingenuous romantic drama “Me Before You,” it’s He’s inspired by the words of Stella, but he’s also been a circus performer for so long, stuck inside this mall (the real Ivan didn’t go outside for 27 years), that he’s withering away.