Why don't they just...
...take these kids to a morgue and say "This is where you will end up if you don't stop." ?
I've never fooled anyone. I've let people fool themselves.
Because people who are at the morgue could be dead for a different reason--diabetic shock, heart attack, car accident. They didn't necessarily cause their situation.
Also, staring at a dad body with no interaction or reaction might not have an effect.
When the kids go to jail, a place they think they can handle and are not scared of, they are interacting with the intimidating prisoners. That's scary to have them yell that crap in the kids' faces.