One well - known creepypasta kid story is 'Jeff the Killer'. It's about a young boy who becomes a deranged killer with a grotesque appearance. His pale skin and permanent smile are very creepy.
If we assume a general plot for this story, one of the scariest parts could be when the kid realizes that the strange things happening around them are not just due to their mono - induced delirium. When they start to understand that there's something truly malevolent at work, it's a terrifying moment.
I'm not entirely sure which specific 'Creepypasta kid with mono' story you mean. There are many Creepypasta tales out there. It could be about a kid who has mono (infectious mononucleosis) and then something very strange or terrifying starts to happen to them, like seeing strange apparitions or experiencing abnormal events. But without more details, it's hard to tell the full story.