Watching the film doesn't make the decision for her to play the child version any clearer, I have to say. I can only assume it's so that the scenes with adult Rene are less weird since she's got an adult face* and he's not acting against a child, but that's how it was in REAL LIFE! Also he's called Jean Charles or something like that in the film, I'm just calling him Rene because come on. The use of the same face throughout also makes it hard to follow the timeline; they stop doing the CGI face stuff when she's around fifteen I think, but it gets quite confusing.
*(And I'm really not trying to be mean, but she looks even MORE like a fiftysomething woman in the child scenes. She has a whole baggy/jowly jawline thing going on which isn't there when she's playing the older version! Maybe it's some sort of CGI error.)