What do kids with cancer and dark humor have in common? They never get old.
Cancer is a serious disease, you realize this, correct?
It's a joke. Not everyone lives forever, even cancer patients don't.
You are correct in that we don't all live forever, but to mock those who suffering from cancer, especially children, is immoral and insensitive. Dark humor doesn't always hold up and you should take more care with your words, as they might make those who are fighting for their lives to feel discouraged and lose hope.