I've just seen the girl's mother being interviewed on Brazilian TV (Fantástico - TV Globo). She said that the girl's seat missed the belt (that connects the restraint to the seat). And also that she had asked her daughter to check wether the restraint was locked or not, and the girl had done it at that moment.
If it is really true, park maintenance can be blamed. But if it is so easy to be broken, maybe a more resistant belt should be developed by the manufacturer.
Well, the case is still under investigation.