It’s an interesting question. Legally, you can give a microphone to anyone. Ethically, you probably have some responsibility for the message you are amplifying. On the other hand, Dr. Curry has sometimes made a sort of a “serving kids alcohol” argument about this — that deniers are going to say these things, and swap these slanders back and forth, and it’s better for it to happen with some adult supervision.
Not a clear-cut ethical question, to be sure.