Oh boy, this is adding gasoline to a fire doing this topic
First of all, even though I consider Kopa cannon in the wider scope of TLK universe, the fact is that TLK creators consider Kopa only to be cannon in TLK6NA, When TLK was made, the cub at the end of the movie was referred to as Fluffy and not given a gender. The creators did not think TLK would be the huge success and well force it is today. I think Fluffy gave most people an impression the cub was a male since Fluffy looked like Simba as a newborn cub. Let me just say this when SP came out even though I was young I still thought "Why is there two births? Why are you (Kiara) being born twice?" Officially, Kiara is the firstborn cub of Simba and Nala. I think it was a moronic move to say that Fluffy is Kiara when they do not look alike. For SP they should have just said left Fluffy's fate alone or made Kiara look more like the cub at the end of TLK.
Now outside of cannon, I believe the cub at the end of TLK was intended to be male by the creators who changed their mind by the time SP came around and decided to make it based on a female lead. However, the idea of Kopa in TLK6NA or Tanabi as COTPL states is not really that far fetched of an idea. I believe in Kopa being cannon making him the first-born cub of Simba and Nala. No, that doesn't make it official as Kiara is the official first-born cub, but I believe the idea of Kion came from the fans wanting Simba and Nala to have a son based off the feeling that the cub born at the end of the TLK leaned towards being a male (Kopa or Tanabi in some cases).
Most people are not going to read TLK6NA or the comics so the movie verse will be the official timeline and cannon. This being said the cubs of Simba are Kiara (first-born) and then Kion.
If you take TLK6NA, comics, and other sources (such as a German version where Kopa is present) then it is Kopa, Kiara, and then Kion. In the earliest drafts of SP, Kiara had a brother so this also shows that TLK universe has been moving towards Simba having a son for quiet a while.
Dog Mom to Misty, Sango, Tami, Abby, and Kawaii.
Three Angel Babies at the Rainbow Bridge: Mija, London, and Olga