I loved The Babadook. I watched it 2 weeks ago and I'm still trying to take it all in. What I can't seem to understand is, what's the significance of Amelia's toothache or jaw hurting? Did I miss something?
The whole concept of disease or illness is often related to emotions which have been repressed. When a person holds in anger, that angry energy has to go somewhere. Some people hold it in their jaw, others in their chest and some in their stomach. Angry energy can actually be held anywhere and everywhere in the body. This energy, if not released, then does violence to the body itself, in the form of disease. So the person that holds in their feelings and does not say what needs to be said, may experience tension in the jaw which can result in TMJ or grinding of the teeth.
You see that in other horror movie like Secret Window. The main character there also displays symptoms of TMJ.
Regarding it not all being in her mind and her son seeing the book too: I am going to rewatch that scene where they first read the book together because another poster mentioned that the book changes which could mean the book was also all in her mind. This would mean the whole thing is her own mental illness which she has to keep controlled (looked after by feeding it and soothing it and keeping it locked up safely and contained in the basement). Her son is close to her and on some level understands her mental illness which is why he talks about it (the babadook) too. The state of the house and their own appearances also represents the state of her mental health which hugely affects her son as well.
That explanation works well with my whole theory. She's repressing anger. Extreme anger. I mean her husband died because of her son (in her mind). In the beginning it shows her grinding her jaw. As the "monster" becomes stronger you start noticing her jaw pain. She jolts when she eats ice cream, implying that she chipped a tooth. She creates the monster in her mind. She has extreme sleep deprivation and the monster represents all her anger and frustration. After she realizes that her neighbour exists and says, "I love you both" and her son says, "you're not my mother" and "I know you don't love me, but I love you" she snaps and takes control of her emotions. She keeps them under control, and feeds it (watches after it). It's a British movie, come on. They even say things like look after your temper, mind your temper, etc. Sorry. That was a rant and there are many cool theories about the movie. I'd probably go crazy if I was a single mother with an autistic child that was born on the day that my husband died and I didn't have any release (masturbation scene). I'd at least getting my teeth.