The night that Cash tells him he's going back to his old life, Jack sits and watches his children sleeping. Essentially, the children are going to be DEAD in the morning! They will cease to exist, and Jack will be the only one to remember them! Even their own mother won't remember them! Has to make you wonder what kind of merciful God would give him a lifetime of grief as a "gift", particularly since he has no one to share this grief with.
Unless destiny comes into play and Jack's children are destined, in some form, to be born to him and Kate in their actual lives. OR Jack slipped into an alternate reality and he's just going back to his normal one while the Jack of that world returns.