[David O. McKay]
President [N. Eldon] Tanner referred to a question that had arisen as to the possibility of children who have themselves been sealed to their parents or who were born in the covenant attending a sealing where other children are being sealed to their parents. As an illustration he mentioned cases where parents who have children born in the covenant go to the Temple to have sealed to them children whom they have adopted. Under these conditions the parents would like to take their family with them to witness the sealing of other children. He stated that in two or three cases we have authorized this procedure, but that the Temple rule is that children under 21 years of age and only those over 21 who have had their endowments should be permitted to witness these sealings. President Tanner said that he and President [Hugh B.] Brown had discussed this with Elder Howard W. Hunter and it was their feeling that it should be a general rule that children under 21 may, where they desire to do so, and obtain the necessary recommends, attend and witness the sealing [of] adopted brothers and sisters.
[David O. McKay diary, Aug. 31, 1966, in Anderson, Devery; The Development of LDS Temple Worship, 1846-2000: A Documentary History, http://amzn.to/TempleWorship]