I guess if a child is spouting nonsense in public, they’re fair game for education. If a parent is going to teach a child lies, they should also teach the child to keep the lies a secret if they don’t want to be “corrected” when in public. Children will say anything their parent tells them to anyone at all, unless specifically told not to.
So I guess I might engage in a dialogue with the child. I could try reasoning with them, but I doubt that would work, since people who believe these kinds of things tend to be people who argue from authority, not data. So if you tried to ask the child why they believed it, they’d say because their parents told them so. You’re not going to get anywhere unless you are providing a long term education, and no parent who believed that kind of nonsense would put their child in a school that taught science.
So your alternative is to tell them what they have learned is a lie and isn’t true. But then they have to decide whether to believe you or their parents. Guess who will win that fight?
Strategically speaking, I think it’s best to do nothing except to talk to your own children and try to explain why someone might believe such nonsense. It’s not the child’s fault. It’s the parents’ fault. Not much anyone can do about that.