William Clayton reported that Joseph taught that when a seal is put upon the father and mother it means their children cannot be lost.
B. H. Roberts, History of the Church of Jesus Christ of Latter-day Saints, (Salt Lake City, UT: Deseret News, 1909), 5:531
Four destroying angels holding power over the four quarters of the earth until the servants of God are sealed in their foreheads, which signifies sealing the blessing upon their heads, meaning the everlasting covenant, thereby making their calling and election sure. When a seal is put upon the father and mother, it secures their posterity, so that they cannot be lost, but will be saved by virtue of the covenant of their father and mother.
To the mourners I would say- Do as the husband and the father would instruct you, and you shall be reunited.
A more complete and accurate transcription of this passage was made by Martha Coray. Unlike Clayton's transcription, Coray's version does not imply that the blessings of sealed parents are unconditionally given to their children.