Apparently children make excellent fertilizer. Mayhap Rick Moranis has been there and miniaturized them. Or perhaps they are so lost that they don't even appear on camera. In any case, one might reconsider bringing children anywhere near that tiny garden.
Editor's note: I've been asked about the third option enough that I have to explain it. It's a reference to The Lost Boys, the 80s movie staring Kiefer Sutherland, Jason Patric, the Coreys and various other heartthrobs. It's about children who are vampires, which do not appear in mirrors or pictures.