If it's union work, there are rules you violate at your own peril. This may not be, but I'd bet dollars to doughnuts there are child labor laws that have to be respected, and also, it's sort of a trope of English voice acting that most young boys are played by middle-aged women, because they have similar vocal range.

And also, getting an episode done is being locked in a recording booth for many hours straight. Getting a movie done would be much the same. There's no reason to believe that Adrien would be able to do that, especially with akuma-related things.