I am strongly against the death penalty, so I dont want to imagine a scenario in which this happened. Maybe there should be regulations in place making it so that only registered groups can start donation campaigns. Its a legislative failure. So many crimes are legislative failures...

I think the existing penalties would be deterrent enough. The problem is the criminals know they’ll probably get away with it.

That's probably too harsh, unless it can be proved without doubt that exactly and only due to this X (many) children died and would (with high probability) live otherwise. Thats China level of dealing with similar situations, plus probably harvesting organs for additional profit afterwards and sending bullet receipt to the family.

There are many existing punishments - go after all their wealth, family and connected business, trusts etc. Simply ban them from western financial world. Publicly shame them and make their name a curse to spit on. Properly harsh jail and making sure all inmates know who arrived, I wouldn't expect kinder treatment than pedophiles get. And so on.

How about if they were deliberately given cancer via some means and then legally barred from receiving any treatment for it?

Since that’s what they did to their victims.