Your son is right up to a point; what could be more tragic than putting the sinless Son of God to death? Even Pilate, the Roman governor, publically declared, "I find no basis for a charge against him" (John 18:38).
But what appeared at first to be a tragedy was in fact a triumph -- because by His death on the cross Jesus Christ purchased our salvation. We are separated from God because of our sin, and because we are sinners we have no hope of eternal life. God is pure and holy, and even one sin -- just one -- would be enough to keep us out of heaven. But by His death on the cross, Jesus Christ became the perfect and final sacrifice for our sins -- and because of that, heaven's door is now open to all who put their faith and trust in Him.
And that's why we can call the day on which Jesus died "Good Friday." In fact, it was the greatest Friday in the history of the human race, because on that day Jesus Christ won our salvation. At one time, the Bible says, we were "without hope and without God in the world" (Ephesians 2:12). But now we have hope, because Christ died for us.
Today, as Christians around the world celebrate what happened on that first Good Friday, may this truly become a "Good Friday" for you, as you open your heart and life to Jesus Christ. "To all who received him, to those who believed in his name, he gave the right to become children of God" (John 1:12).