This is an early version of the show. Yes, that is Jeremy Jordan and Laura Osnes. No, the actors who play young Bonnie and young Clyde are not the ones who played young Bonnie and Clyde in the final version of the show/ cast recording. Some of the costumes and lyrics from this version were tweaked later.
Why are people saying young Bonnie is tone deaf? I thought she sounded really good, maybe I don't know the song well enough yet, but I've watched it like five times and I still didn't hear any glaring errors.