I believe the answer is that it is harder and it hurts people more to watch children suffer than adults. When adults suffer we don’t understand that Hashem has a point or a plan and just say “that’s life, these things happen”. But when a child suffers it is not “bderech hatevah” and therefore we notice and it is so much more painful to see a child suffer than an adult because they are helpless and need others to do for them. We are crushed and FEEL the need to to for them more than we FEEL the need to do for adults for whom we make excuses for “they will manage; they did this to themselves; their parents will bail them out; and so on and so forth. But when a child suffers it does not have to be one of your own to break your heart and initiate a reaction to help.
So when Hashem truly wants to send us a message that we don’t seem to be receiving or accepting, sometimes it is the children who suffer to bring attention to the issues around us. When we see young people struck by illness or unfortunately death, the neighborhood gathers together and tries to make some kind of tikun or take upon themselves another mitzvah in the child’s zchus.