This is an explanation of the "two Jacks," from the Mirvish review. There's an alternate for Young Jack, but there's also an adult alter-ego:
"There’s also the issue of Jack. The stage version of Room has two Jacks; and for those who’ve not read the book or seen the film, it’s not immediately clear that it’s two versions of the same person, and not two separate children mothered by Ma. Young Jack’s played most nights by the wonderful Lucien Duncan-Reid, a little firecracker, a Paw Patrol veteran, and an actor with nuance and stagecraft beyond his years. Duncan-Reid, a grade four student, is simply magnetic, and when Ma disappears for much of the second act, he carries Room squarely on his four-foot shoulders.
SuperJack, on the other hand, is a more difficult task. SuperJack is… an adult version of Jack? A five-year-old Jack in an adult’s body? Brandon Michael Arrington offers an animated, compelling performance, but it’s a tricky role, which, as eventually becomes clear, represents Young Jack’s older alter ego. SuperJack narrates events as they happen using a five-year-old’s lexicon, and that choice only works for so long before growing tired. While SuperJack offers a lifeline to the actor playing Young Jack (Levi Dombokah for matinees) who is onstage for Room’s two-and-a-half-hour-plus-intermission runtime, the role often feels dramaturgically unnecessary. I’m glad Duncan-Reid and Dombokah have an onstage buddy — I’m not convinced they need one."