Paper 47--The Seven Mansion Worlds--which describes the probationary nursery
Paper 49, Section 6--Terrestrial Escape--Paragraphs 11-14 also discuss the probationary nursery.
You can also do a search of The Urantia Book on our site (look at the top right corner of this, and every page on our site for the "search" feature) for the 'probationary nursery.'"
Thanks so much for this question. It is of great concern to many, so I am glad you wrote in!