isn't 그 'that' ?
Usually! In this case, the 그 is referring to a girl that people are already discussing, just like "the" works in English, so "that" or "the" are both correct.
In a real situation, you might just drop 그 여자아이는 altogether.
But translating from English "the" doesn't need "그" so you could just drop it and 여자아이는 would work, wouldn't it?
I thought so too, but im not sure.