Well, I agree, if Littlefoot spoke that line, it would make scene more emotional and understandable.
But wasn't it obvious already that his reason to break up with gang is to find and save his father as quickly as he can, despite dangerous path? What else reason he could have?
When I first saw that scene, I kinda felt vibes of first movie, actually, but this time, Littlefoot and Cera changed places. Yes, that's absolutely different case this time, but we again have two different paths, and again Littlefoot and Cera choose different ways.This time, unlike previous case, there's huge possibility that both paths will end up in the same destination, but one path is very dangerous, other one is more safe.
This time, Littlefoot refused to listen voice of reason, Cera had a point after all. Maybe, this scene was even better, if after Littlefoot spoke that line, Cera would counter with "You won't help your father if you die on the way", or something like that. I believe they both would stay with their characters in this case.