As long as it’s explained properly, thoroughly, and fairly, I do not think children should be protected from truths about their parents. I think 6 is old enough to understand almost anything as long as it’s put in the most basic terms, and the child is reassured that he is ok, that the parent/person in question still loves them, and what’s happening is not a reflection of them or their actions.
The problems I have with the urge to shield children from this sort of thing is that it preserves an ignorance of facts that can come back to worsen the situation in the future. In past threads, some parents have not told their kids about their father’s cheating or their mother’s lying out of a genuine desire to “protect” them or to not “harm” that child’s relationship with that parent. Then ten years down the line, because the child was kept in the dark about the crappy parent, the kid doesn’t understand why there is so much anger and bitterness between his parents and takes sides based on his incorrect perception of the situation. At that point, the non-crappy parent either has to endure the pain of the kid being on the side of the crappy parent, or admit that he has been hiding things from him all these years.
I don’t see the point of this. As painful or difficult as it may be to explain the unfortunate truths about irresponsible parents, it’s far worse, IMO, to make up and sustain an illusion about such parents.
If you’re not sure whether your kid has a concept of something, there’s nothing wrong with asking them if they understand and then explaining it carefully if they don’t. As smart as they are, kids don’t magically develop concepts of things on their own. With difficult things like their parents being in jail, wouldn’t it be better to sit the kid down and explain to him where Daddy is and why he’s there, than to conceal the truth and possibly not have control over how he finds out about it? It would be a greater shock, IMO, for your son to find out about it when he’s 25, or to hear it blurted out at a family gathering next Christmas.
Since his father wants him to know, I would have him write a letter to your son explaining in his own words where he is and why he’s there. You can read it with your son, and then be there to answer any questions your son may have. It’ll be a lot for him to digest, but I think reading it in his father’s own words will help him understand and accept it.
With something like lawbreaking and jail, I think it’ll be important to emphasize the difference between Daddy breaking the law and going to jail and him (the child) misbehaving and being punished. After all, the idea behind telling him the truth is not to make him paranoid that he could be taken away if he misbehaves.