Joseph is reported by William Clayton as teaching that when a seal is put upon the father and mother it means their children cannot be lost.
Joseph Smith, Discourse, 13 August 1843–A, as Reported by William Clayton, p. [87], The Joseph Smith Papers, accessed September 8, 2021
When speaking of the passage “I will send Elijah the prophet &c” he said it should read and he shall turn the hearts of the children to the covenant made with their fathers Also where it says and they shall seal the servants of God in their foreheads &c it means to seal the blessing on their heads meaning the everlasting covenant thereby making their calling & 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.
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.