I think this is a fairly balanced article about the whole business
www.snopes.com/news/2019/02/26/momo-challenge-suicide-game/
AT 6, I really think a general internet safety discussion would have sufficed (never talk to someone you don't know over the internet, never talk to someone over the internet without your parents' knowledge, never give out any of your contact details to anyone you don't know or trust etc).
DD is 12 and this came up with her friends. She was beyond freaked out by the picture (shown to her by a friend). She was v much comforted by knowing that it's a sculpture created by a Japanese special effects company not for this purpose and someone has just stolen the photo and reused it.
We had a chat about it and I pointed out that a) she'd done the right thing by telling me, b) always tell me if something is scaring her as otherwise I can't help if I don't know, c) it is almost always true that adults are likely to have a better handle on whether things are actually worth worrying about than 12 year olds and it's still worth asking an adult even if you think they might not know and it could be another trusted adult if for some reason she did not want to ask me, d) 12 year olds are not well known for their ability to be calm and rational in the face of something scary (this made her laugh quite a lot) and e) just don't ever talk to people you don't know over the internet and nothing like this can ever happen to you (in the case of someone you don't know talking to you without your permission, the correct course of action is always to tell me immediately and block that person).
It does seem like it's a lot of fuss over nothing. I expect the school were trying to be proactive in protecting them but you're right that this is a level of detail the vast majority of them will simply not need. In the case of 6 year olds who do have unsupervised access to the internet, I suspect Momo is the least of their problems. I suspect a general 'most of this stuff is absolute nonsense and if you see anything that scares you, you should tell an adult you trust' would have sufficed for 6 year olds.