Wow, that is REALLY similar. No, I hadn't even heard of the movie before you (and someone else on AO3) mentioned it.

I actually got the idea from horror movies--you know, there have been a couple where little kids had imaginary "friends", but they ended up being evil/scary. Haha, that sounds like such weird inspiration, I know, but I was mostly just thinking of the idea of an imaginary friend who ended up being something more.