Jack has a kid in one timeline, his name is David.
In the other he is not born and not named, but according to me, exists like everybody else and appears as black smoke.
But everyone who exists still has the same parents. Jack still has Margo and Christian, Sun's dad is still Paik, Ji Yeon is of Sun and Jin, but who would the parents of the smoke be? Let's just pretend that David's mom is Juliet. How in the island timeline will Juliet and Jack have a child and then end up being ths smoke? I'd like to be able to follow this theory a little bit better.
In my head, it ends up smoke, because it isn't born. The events in the crashtimeline are manipulated in such a way by Jacob that David Shepard is not born and has no body. But as he is supposed to exist, because he existed in the original not-manipulated non-crashtimeline, his consciousness or soul is there. (They spend an entire episode teaching us about timetravelling consciousnesses).
The black smoke knows he is supposed to exist as a body too. I think he has the bodies of Jack and his not shown mother (maybe Libby) from the original timeline in the cave. (Adam and Eve)
That consciousness wants a body. He now can only use empty bodies of people that died, like Yemi's and Locke's and maybe Christian's.
But he is trapped on an island where no babies can be born since I believe the day David Shepard was supposed to be born (in 1977 Ethan was born).
To answer your question, I don't think Jack's kid turned into smoke after being born in this timeline, it just isn't born in this timeline and Jack needs to give him his body, because he now only is soul or thought or consciousness. Jack needs to ignore Jacob and bring MIB to the other timeline where he has a body and a house or let's hope he impregnated Kate the day before they returned to the island.
You only have to assume that MIB is not evil and that the sidetime is the original one where he wants to go and everything you watched the last 6 years will fit.
From Sarah leaving Jack because of Gabriella before he could have a kid to Jack not supposed to raise HIM (Aaron) to Christian bringing Jack to the cave where Adam and Eve are to John Locke sacrifising himself to become MIB's body and seeing a beautifull light to why Eko was bad to the meaning of all the titles as raised by another, homecoming, there is no place like home, abbandoned, eggtown, maternity leave, the meaning of live together die alone.
The meaning of all the parent-kid issues, all the bibblical references to sacrificing the son, the meaning of the land of Oz and the land of Alice, that both return to their original timeline and their sweet home in the end. All the references to books like Ullyses returning home, the bunnies that want a home and George and Lenny wanting a home. To the picture of MIB in the center of the table in the lost supper. To MIB's sentences as "once we are on thet plane whatever happens happens" to Widmores if that thing leaves the island we all cease to be. Why no new life is allowed on the island and why Smoky needed pregnant Rousseau and Jacob ordered to kill the baby. And watch how they cast kids. Look at young Ben and adult Ben, look at young Kate and adult Kate, look at young Hurley and adult Hurley and than look at young Aaron and Jacob and David Shepard and the Man in black. Also why they showed us that it is possible to travel to the past (1977) and probably it is possible to change things that happened and manipulate people into doing things (Did Sayid change Ben? Did Chang evacuate the island, because Faraday told him?) and why they told us about timetravelling consciosnesses.
Just try to think as MIB as being the good one and the island as a prison and everything we watched makes perfect sense.
If Jacob is the good one, I still cannot see the final picture, that the hundreds of puzzlepieces they gave us, have to make. The only final picture I can see is them all being alife in the original timeline, Locke marrying Helen, Jack and Kate raising David, Claire and Charlie raising Aaron, Ben teaching history and taking care of Alex and his father, Hurley a nice, lucky man finding Libby, Sun and Jin having Ji Yeon and raising her together from the start, Sawyer finding Juliet and accepting his past, Michael raising Walt, Jack operating Locke, Desmond happy with Penny, Keamy dead, Sayid marrying Nadia, Widmore claening toilets in Ben's school and the island deep under the sealevel.
And I don't buy that people that make such a wonderfull show, made a mistake by making Smoky kill Eko, because the actor wanted to leave the show. They totally know what they are doing, from episode 1 where Smoky killed the man, that wasn't supposed to fly the plane, but brought a new group of people to that awfull island to die or loose everything there.
