The first and the second is manga, why the third becomes an exception - the movie?
By WHY, do you mean why I think that's case, or why should Nakaba do it? I answer both of them though!
1. Based on FKS's post, Diane is searching for her children. If they were gonna be absent from the movie, there was no need to put those lines in early promo material. The movie would be enough to raise anticipation. Why should they specifically leak such data if the movie is not gonna provide more useful information? Not to mention that GoE is a two part project.
These four kids might be Kiane's or not, but it's highly possible that they will appear in the movie IF NOT APPEAR EARLIER IN SEQUEL MANGA.
2. Why should Nakaba choose a movie for thier debut?
The only person who can answer this question with one hundred percent certainty is Nakaba himself, but we can guess why should he be doing that IF THAT'S REALLY THE CASE.
First of all, let's make a more practical distinction. We have two types of dualities here:
a. Main story vs Side story
b. Manga vs Original anime
Among the most prominent characters debuted in a side story are
Escanor,
Zelda, and
Lancelot (One may count Dahila and Dabuzu as well). Kiane kids may get added to this list if we see them for the first time in NnT: GoE.
There could be several reasons why they should appear in a side story. For example, if they are gonna have significant ties to Trisran, a side story in which Tristan is the protagonist is a better place rather than a manga featuring Percival as the protag.
The relative independence of a side story also makes it a perfect place to start a side plot in complex stories like NnT and 4Kishi. Tristan can find a lover or some close friends in GoE and appear alongside them in 4KotA without causing any confusion or any kind of unneeded mystery! And if any of Kiane kids happen to be among those characters, the perfect place for their debut would be this very side story.
Why should it be an anime project instead of a manga chapter/one shot. Nakaba is a known author and has a legendary studio the almighty Marvy Jack by his side. They are alway ready to shit on Nakaba's work and turn it into an animated shit!
A little extra money is spend compared to a simple manga chapter, but more people will know about continuation of NnT after watching young Tristan in it. It will ultimately be more effective in the long term and increase the income.
Long story short, anime or manga is a question of finance and marketing more than anything else, and it has very little to do with the artistic aspects of characters, their first appearances and their role in the story.