I'm writing an episode which involves a plot by the monster Vanish involving possessing the dark side of the moon (if you don't know, his power is to possess shadows). I was going with the fact that the term "the dark side of the moon" would cause enough children to believe that it really was dark for Belief to make it so, but I'm not entirely sure that Belief works that way. Does it?
It's been a long while since I played Little Fears and I never did GM it, but from what I remember Belief was a child's main defensive thing. It also had it's negative side such as holding your breath when riding through a tunnel, or lifting your feet when riding over rail road tracks, or not stepping on cracks in the sidewalk. All of which have some form of negative consequence which can have a regional component in some cases.
Judging by your wording of the question, "would cause enough children to believe that", it sounds you may want to impose a threshold of some kind on the plot. From what I remember, you don't need to go that far. Only one believer would be needed to make it happen. But children that didn't believe, say because they never heard of the possibility, would not be subject to the effect. The beautiful thing is that once a group of children hear about something, some of them will believe. A real life example of that being a young sibling of my friend believed I had a battery in my big toe. Why? I said I did. It was that simple. In the same way, agents of Closetland could spread negative believes that serve one of the seven kings or whatever master the agent follows. If you get to them young enough they will believe some really wild stories.
Have fun with Little Fears, I may have to go looking for it again.