About 2-3 years ago I saw a documentary on PBS about a mother who takes her son to a cross country journey to visit her husband (and his father) in prison. I forgot the name and was not able to find it in their site.
Anyone knows a site or a way to figure it out? (please don't write IMDB).

That sounds like one of the episodes of the PBS documentary described below:

"A new episode of Reading Rainbow, “Visiting Day,” aired on December 15, 2004 and again on January 17, 2005. In this episode, LeVar introduces a family separated by a prison sentence. The viewing audiences join the family for visiting day and find out what life is like for kids when a parent is incarcerated and what it's like for a parent who can't be at home with his family."