What would you say to someone who is afraid that a bad actor will find their kids on Discord/Minecraft/4chan and encourage them to commit suicide or shoot up their school?

I would tell them to not give their children internet access at all until they think they are ready. Does it sound realistic? I don't know. From technical point of view certainly realistic, a child doesn't have money / devices of his own

Listen to your children, give them attention, they are your responsibility, don't give them internet access if you find it unsafe