Well, new information reveal that although the three kids were minding their own business at that time, this incident was indeed the pursuit of a previous one between two 'cités' (suburb cities).
Apparently, it all started with one guy being hit at the head with a... hammer, days before that. They retaliated with machetes. This is life around Paris.
For information's sake, notice that the ethnic and religious background of the protagonists has not been disclosed yet - as far as I can tell. Therefore, I can't tell if they were Muslims, Christians, Buddhist monks or plain atheist thugs.
That said, considering the area involved and the very fact that these details about the attacked and attackers WEREN'T disclosed (usually a good indicator, considering the utter PC stance of the French press), we can probably drop the "Buddhist monks going amok with machetes" option.
These guys are too easy to trail with their orange robes, anyway.