Steady on. They (Maddy's parents) took a chance that the kids would be OK left sleeping in their beds. What's that odds that something bad would happen? 1000 to 1? 100,000 to 1?
Are they likely to ever fuck off out for evening and leave their remaining 2 kids alone again? No.
...so what good would taking the kids off them do? Would the kids be better off? No.
Those parents made a terrible mistake, one that will haunt them for the rest of their lives. Sending them to prison would serve no useful purpose and would be unfair to the surviving children.
As for you hoping they "rot in hell" I think they will already be in a kind of hell for the rest of their lives, regretting what they did that night. That's more than enough punishment, in my eyes.
nb
Although they aren't doing themselves any PR favours with the 'private jet to see the Pope' business IMO