Yet Another Major Plot Hole
Mathilda spends several months with Léon. During that time, she is not far from her home, and not in hiding. Throughout that time, nobody seems to be looking for her.
First of all, either the police or the CPS (Child Protective Services) or both would have been made aware that there is a missing child in that family, and would be making efforts to find her.
Secondly, a family in which 4 people were gunned down in their apartment would have been major headline news. There would have been photos of all the family members -- including Mathilda's. And the articles would have stated that she is missing and that anyone who has seen her should contact authorities.
So Mathilda in reality would have been spotted within days of the murders.
It should be against the law to use 'LOL'; unless you really did LOL!