The problem with this is, Lando and dot say that it was only for laughs, but treats the tragedy as a joke as well. The joke wasn't funny though. (Just saying, imo.)
People aren't really overreacting, but it's just difficult to grasp that people are making fun of something that was so depressing. Of course, the world will not change, so yes, you can call them idiots, but it's only making them laugh more.
My point is, the joke wasn't funny in my opinion, but I can see where you're coming from, because I laugh at some jokes about tragedies as well. I think the main problem was the victims being children younger than 10. People shouldn't think that a 7 year old dying because of a mentally ill person is funny, but as I said before, the world won't change.
It's best to just drop the subject.