I would be responsible parent and explain what happened in terms that suit the child's age and under no circumstances allow them anywhere near them?!For a start you wouldn’t want your image plastered all over the web in their Vlogs where it could be falsely interpreted as supporting or associating with them.
I don’t know what I would do if my kids wanted to meet up with them. I’d be loathe to punish their children for his actions but I’d be all over controlling that meet up to protect my kids. Supervised access in a contact centre is where I’d want a fan meet but then again it’s a good opportunity to reiterate to kids that just because it’s so and so’s dad and they’re a YouTuber doesn’t make them a good person but that doesn’t make his kid a bad person either.