It is not a war crime to fire back at terrorists or even an ordinary army fighting in a war if they are in a civilian area. The war crime is actually commited by those firing from a civilian area. Israel doesn't have to sit idyly by while its civilians are targeted. She is entitled to fire back, though there is an obligation, where it is possible, to take care so as to not hurt civilians. However, where, as in this case, the terror cell is firing in the middle of civilian area, there is no aboility to take care.
4 UN peacekeepers injured in Sinai and taken to hospital in south Israel (Haaretz)
from the article: IDF strikes Gaza site after mortar shells fired into Israel