First things first; Kubrick is (was) a far more distinct character than an ordinary Hollywood figure. A reclusive, he is said to have retired from Hollywood industry (but not from cinema altogether) very soon and to the UK, as far as I could know, his own country. He is also reported to be somewhat obsessive, eccentric, wherefore pretty snobbish, which is not up to me to discuss here.
I don't know the guy, but I know a couple of his works to understand that he had a quite clear notion about what he was doing and what he wanted to do. The opening of the film is both aesthetically and philosophically a triumph; a peak that viewer is scared not to be able to feel the same emotion in the remaining part. Flying over some natural wonder, a beautiful scenery a beautiful music, bronzing trees, still lakes... Beauty is aesthetic in itself; but that much beauty leading an eerie and uncanny atmosphere is philosophically paradoxic: so beautiful that one could hardly believe such beauty could lead to such horror. Kubrick, in fact, loves dealing with duality and paradox: Remember Private Joker, "Born to Kill" written on his helmet whilst a "peace button" on his chest; answering the commander: "Duality sir, the Jungian thing!" This duality is also significant in the using of mirrors: A mirror shows what is in front. But in fact a mirror degrades (or reproduces) things. A mirror does not show what the thing in front really is but rather, to quote Sylvia Plath, "A mirror shows what you see". Since we are mostly symmetrical we can't get to understand the deception. It must really affect us in a profound way to grasp it: Either you have a cut on your cheek while shaving; or you wake up and see the reflection on the door: redrum, therefore, murder! Preferably the first. Mirror is the most poetic way to describe the duality and the paradox. A broken mirror very likely represents a wounded self or an inner break-up, nonetheless, in this case an inner integration: the very moment Wendy (Shelley Duvall) sees the reflection on the mirror, she gets determined to save herself and her child. And another duality becomes distinctive in Danny's (Danny Lloyd) imaginary friend Tony, who lives in Danny's mouth: Inspiring clairvoyance, a sense of reality and wisdom to certain degree, briefly an adult inside a child. Especially in the beginning and then again a few times until the end he insistently repeats: "I guess so!" That is quite an answer for an 8-year-old. Inexperienced about life, an infant is barely conscious of this ignorance; he/she is generally stubborn about his/her knowledge and barely able to accept it. "I guess so" nevertheless, is an answer likely to be provided by an older human-being; who has seen so much in his/her lifetime and gotten so much confounded by its consequences that he/she is merely able to retain an arguable certainty. So we could say that Danny is almost a senior citizen, since he has seen a lot, both the past and the future, especially considering the peculiar circumstances he is under.
Let's focus for a while over the circumstances: writing a horror story is much related to creating the right ambiance. Agatha Christie in her 'Orient Express' was able to create a horror fiction which is unfolding in a train.