I learned that those who are worthy who didn't marry in this life will be ministering angels to those lucky ducks who are sealed to a companion. But then I also learned that worthy women would have the opportunity to be sealed to a worthy companion in the next life. The usual take your pick whatever keeps you active doctrine.
Going by the BoMor and JS, my parents taught us that my baby sister who died would be an angel. But they were wrong according to subsequent prophets who apparently give my sister a chance to grow up and meet an RM in the millennium.
Which makes one wonder, why waste all that paper and ink on all the doctrinal stuff that said it was too late to marry after you carked it, without mentioning the millennium exclusion clause?
Short answer: The living prophets were plucking it out of their arses, without the search and index capabilities we take for granted on digital documents these days.
Actually, the new improved, grow up in the millennium and marry an RM doctrine so completely supersedes the old just-an-angel doctrine, that it is hard to understand why anyone would even bring up the angel option as the "standard" at all.
This is just another example of pulling ridiculous feel-good doctrines out of Joseph's arse.
The first babies-as-angels doctrine helped to console the suckers at a time they needed comfort following the loss of a child. No further was needed, as no further doctrines were believed at the time.
After creating the "exaltation" doctrine as a reward for sexual favours, and as a reward for like behavior by male associates to ensure their compliance, the angel doctrine didn't cut it anymore. So it was left to subsequent prophets to extend to blessings of celestial marriage to the dead children.