If Israeli families had zero tolerance for making martyrs out of their youth there wouldn't be any soldiers in the Israeli Defence Force. Instead, every Israeli teenager is conscripted into the military except for those who are exempt on religious, health, disability or criminal grounds. 

And yes, it's not always the case that a rudimentary rocket from Gaza falls harmlessly in the desert. On relatively rare occasions they do injure or kill Israelis. According to the Israeli human rights organization,  B'Tselem, "From June 2004 to 23 July 2014, 26 Israeli civilians (four of them minors) and two foreign nationals were killed in Israel by Palestinian rocket and mortar fire."

So it's not the case that Israel has no justification for responding to those attacks. What Israel isn't justified in doing is responding with such force that 100 times as many Gazan civilians are killed. This is true both morally, and also according to the laws of war.