I can confirm that. Had a similar situation in the end of the last year (paid tickets on LA, not JJ). The child's ticket was just a little bit cheaper, but it was sold under some sort of "child / discounted" tariff. Didn't quite paid attention when purchased it though (since the difference was really small). When it came to posting upgrades I was quite surprised that the child couldn't be upgraded due to that very tariff issue. Called LATAM and two different staff members confirmed that the child's ticket is not eligible for an upgrade indeed. The worst part - the flight was not full at all, there were plenty of empty seats in business
Lesson learned.
These are the corresponding rules:
https://www.latam.com/en_us/plan-boo...nd-conditions/
The child's ticket falls under the exclusion "passengers traveling with reduced fare tickets", this is how it was explained to me. The fact that this very ticket might be just 10$ cheaper doesn't matter. It's all about the definitions.