Instead of me telling you, read the story. You'll thank me for it later. (Seriously, it's wonderful.)
I did read the story, and (maybe) I get it now. The only thing I can come up with ist that they're afraid of Mokou due to her association with fire and the fact that they died when wheir house burned down. If that's the case, then the Mean But Hilarious pool couldn't be more appropriate.
It's not like Mokou's doing it on purpose, though. Her expression reads more like "What's up with those three?" But yeah, you can hardly blame them for being skittish around fire or even just a lit cigarette, considering.
Especially surrounded by wood. If I'd died the way they did, I know my resurrected poltergeist form would be.