Hamas got into an argument with the Ayad family east of Gaza City. Apparently, Hamas wanted to use their home to store explosives or fire rockets, and the Awads disagreed. This resulted in a gun-battle, killing the Ayad's 15-year old son, Mohammed Jawad.
This has only been reported in the Palestine Press Agency website. Other news sources are either pro-Hamas or cannot get press passes to report news from Gaza without harassment; PPA seems to rely more on underground pro-Fatah reporters and citizens, so it is somewhat less reliable. A number of readers in the comments section confirmed this incident.
My count of Palestinian Arabs violently killed by each other this year is now at 573, with 40 of them women and 42 of them children.
UPDATE: PCHR reports that two were killed and four wounded in those clashes. 574.