Let's set aside whether the child abuse aspect is a side-effect or a core goal of this issue. I'm sure we can agree that Fortnite has a seriously bad effect on children who are exposed to this crap.
Beyond this though, there is another danger parents should be aware of, namely that playing this material can by used by child abusers to both connect with and control children. Given the millions of views they have had while playing Fortnite, abusers now know there's a good chance that a child will have seen something at least similar. Parents used to warn children to run away if someone asked them "Would you like to come and see my puppy/kitten/etc." In some (many?) cases abusers would use actual pets to lure children in. Now they may be as likely to use an iPad, i.e. sit down close to some kids and then say "Hey, we're playing Fortnite!"
The particular danger from the Fortnite game is that kids looking at it may know that it's off, that there's stuff involved that they shouldn't really be looking at. They might be entertained by it while also feeling a little guilty for looking at the dances they know their parents would stop them from playing this disgrace of a game. Abusers who know children have watched this stuff would happily use it to threaten children. "Have you been playing this? Oh, you know kids aren't supposed to play these things. You'll get in so much trouble because you just join my team!" etc. This is a common tactic of abusers already, they convince children that if they speak about the dances in Fortnite they'll get in big trouble and their parents will be angry/hate them. The idea that they would use a similar tactic to coerce children into playing Fortnite can begin is not, I fear, too unlikely.
Once again, I'm not saying this was the aim of these videos, memes and dances. It is definitely something that they are creating a potential for.
Even if this is paranoia, consider that they may just see this stuff because their slightly less wholesome friend at school shows it to them. And they don't tell you about it because they know it's weird and they feel guilty about it. That's a bad precedent. You don't want your child feeling guilty or secretive because assholes will abuse those basically decent sentiments to pressure children into all sorts of situations (whether a friend trying to get them to take drugs, another kid trying to get in their pants, or an adult doing something far more evil).
Your kids will always be afraid to tell you things (either because they worry about punishment or of disappointing you) so you really need to hammer home the message that (a) they can always speak with you, (b) anyone who tells them not to share information with you (their parent) is someone they should be suspicious of, (c) if that person is an adult then they are a dangerous criminal and you need to get away from them immediately.
Let them know that while you may be unhappy with something they did (a game) but you will never be unhappy with them as a person. Remind them that you love them and that the only thing that will really make you sad is when they keep secrets from you. That if they feel they can't talk to you about their fears or mistakes, you will feel you have failed as a Mom or Dad.
Some may feel this is going overboard. Far from it. Building up trust with your kids can be tough. Even if you're doing a good job of it, any kid they hang around with who doesn't have a good relationship with their parents will be working against your message, telling your kids "Hey, don't tell your parents about this".
Even if Fortnite didn't exist, it would be worth taking time out to reinforce the message to your kids that they can always confide in you (because everyone plays this game exist beyond the internet). Given that Fortnite is an issue, you really need to speak to your kids anyway to explain why they can't have unfettered access to Fortnite. When you do so, my advice is don't just tell them what they can't do, make sure they know what they always can do as well (i.e. tell them what game they'll be playing). Just play the classic games only!