The Book

I just finished reading it. In the book Maggie also has twin girls younger than Doug.
The Forshay ordeal comes at the end and resolved quicker than in the movie. The author goes into great detail on why Forshay was discharged and why Baggs hates him so much.
John Jr. is still born two months early but is accidentally killed at the hospital by a nurse along with several other babies.
They don't follow Maggie. Baggs stays with the three kids, trades id's with Forshay, and I think he makes a living selling religous books if I understood correctly.
It may have been implied that they'd go after Maggie when Baggs had enough money. I don't recall. But the author does not have Baggs and the kids walking off at the end of the story.