Do parents of children that have been born under the covenant and fall away have a promise from the Lord that they will some day return to full activity? I think Brigham Young has a quote to that affect. Do you know where I can locate it?
The quote that you refer to by Brigham Young, was stated by him as he had heard it from the Prophet, Joseph Smith. From Joseph Fielding Smith’s, Doctrines of Salvation, compiled by Bruce R. McConkie, 2:90-91, we read–
“We cannot overemphasize the value of temple marriage, the binding ties of the sealing ordinance, and the standards of worthiness required of them. When parents keep the covenants they have made at the altar of the temple, their children will be forever bound to them. President Brigham Young said:
“Let the father and mother, who are members of this Church and Kingdom, take a righteous course, and strive with all their might never to do a wrong, but to do good all their lives; if they have one child or one hundred children, if they conduct themselves towards them as they should, binding them to the Lord by their faith and prayers, I care not where those children go, they are bound up to their parents by an everlasting tie, and no power of earth or hell can separate them from their parents in eternity; they will return again to the fountain from whence they sprang.”
Elder Orson F. Whitney referred explicitly to Joseph Smith as the author of this concept. Whitney reported
“The Prophet Joseph Smith declared and he never taught more comforting doctrine that the eternal sealings of faithful parents and the divine promises made to them for valiant service in the Cause of Truth, would save not only themselves, but likewise their posterity. Though some of the sheep may wander, the eye of the Shepherd is upon them, and sooner or later they will feel the tentacles of Divine Providence reaching out after them and drawing them back to the fold. Either in this life or the life to come, they will return. They will have to pay their debt to justice; they will suffer for their sins; and may tread a thorny path; but if it leads them at last, like the penitent Prodigal, to a loving and forgiving fathers heart and home, the painful experience will not have been in vain. Pray for your careless and disobedient children; hold on to them with your faith. Hope on, trust on, till you see the salvation of God.” (Elder Robert D. Hales, With All the Feeling of a Tender Parent: A Message of Hope to Families, Ensign, May 2004, p.88)