EnderCube

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.

GalahadFairlight

Answer: It could be kind of like the thing from the "Wimpy Kid" movies, but with voice acting. The voice actors could be infants or toddlers, which don't behave well. So the filmmakers have two of them (It doesn't have to be twins since they're doing voice acting) that way they can swap out one with the other.

EnderCube

Chosen answer: I believe it's called "This is My Home", sung by Will.I.Am, who voiced Moto Moto.

swandell

Answer: The song that played when Alex and Zuba were dancing is called "The Traveling Song," sung by Will.I.Am.

EnderCube