through Heavenly Father's plan..." This is one of the most popular songs I heard growing up and one that really instilled fears and also motivation in me to choose the right. The most horrible thought for a child is the threat of losing ones family. Talk about damaging a child's security!
In conference talks you will sometimes hear the leaders talk about "wayward children" who have a kind of second chance if you will, at remaining in their eternal family because of the sealing. I have heard many a parent talk about this safety they feel once they have their children sealed to them, or those who were born in the covenant because of this fear of losing them to sin. This makes no sense either because repentence is for everyone.
Lets look at the language of section 132:
26 Verily, verily, I say unto you, if a man marry a wife according to my word, and they are sealed by the aHoly Spirit of promise, according to mine appointment, and he or she shall commit any sin or transgression of the new and everlasting covenant whatever, and all manner of blasphemies, and if they commit no murder wherein they shed innocent blood, yet they shall come forth in the first resurrection, and enter into their exaltation; but they shall be destroyed in the flesh, and shall be delivered unto the buffetings of Satan unto the day of eredemption, saith the Lord God.
27 The blasphemy against the Holy Ghost, which shall bnot be forgiven in the world nor out of the world, is in that ye commit dmurder wherein ye shed innocent blood, and assent unto my death, after ye have received my new and everlasting covenant, saith the Lord God; and he that abideth not this law can in nowise enter into my glory, but shall be edamned, saith the Lord.
So you can pretty much commit any sin except the shedding of innocent blood and blasphemy against the Holy Ghost, and still qualify for exalation. This is how Joseph was able to promise entire familes this reward once they were sealed to him.
It doesn't mean they have fulfilled the covenant, but they are protected from being placed in the other 2 kingdoms by the sealing ordinance. The covenant is not fulfilled until they enter plural marriage, so this merely allows the children a higher status to qualify for exaltation than those who are not sealed.