WWI or WWII Movie Help Please!!?

I have been looking for the title of this movie for 2 years now...

A 8-12 yo boy is the primary character. He has at least an older sister. One of the major scenes is the boy and his gang of buddies catches the older sister having sex and they all start throwing rock as the couple. The boy later on goes to live with his wealthy grandfather where I remember a scene where he played cricket and another scene where he and his grandfather were fishing and the grandfather lit a stick of dynamite to catch the fish. This next scene may be confused with another movie, but the father finally comes home from the war and brings home a can of jam, but the family won’t eat it because they are worried the Germans poisoned it.