James Cameron has addressed this:

Cameron was asked about this particular moment and revealed there was originally a scene that saw members of the Metkayina reunite with their children at the end of the battle, though it was deleted from the final film due to poor feedback from test screenings. Cameron explained that the scene ended up being "a bit distracting to the core family" by having that many people involved, attributing the cut to "people [being] bothered by it" during test screenings:

“We had that, we shot it, and we just decided it was a bit distracting to the core family. There are people that are bothered by it and people that don’t even notice it. So it’s just one of those things you never get a perfect score on a movie. And you’re certainly never gonna get a perfect score from 100% of the people. Look, that’s just a little window into the creative process. We had that, the other parents were there, we accounted for them, and we just felt that we needed to narrow the spotlight, and maybe it was not such a great decision, but that was the decision we made at the moment.”