Other mistake: Judging the angle Teetsi leaps towards Alex at, there is no way that he would hit him in the eye.
Other mistake: When the plane is crashing the two parachutes are deployed at the front of the plane yet the plane remains level when landing.
Chosen answer: There's two reasons. Firstly, infants are only allowed to work for short periods of time, hence why when you see babies onscreen that are heavily featured, invariably twins are used to alternate to not fall foul of the rules. Secondly, it may be that one of the voices didn't give the filmakers as much diverse dialogue as they needed so they brought in another actor to get the rest they needed.
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