Summer of 42 is the film Danny and Wendy are watching during No.40.
It is inappropriate for a child like Danny as it deals with a young teen losing his virginity and getting emotionally involved with, a young-ish (but still absolutely older than the boy) grieving war widow.
There are significant visual choices that are made in the film to connect The Shining with the themes of Summer of 42.
The Boulder Apartment has curtains and the kitchen elements, Jack is dressed like the young boy, and the way his jeans are layed out as Danny enters No.41 is a reflection of Summer of 42 (and the scene where the boy has had sex for the first time).
A stretch? Maybe. But the reflections and overlaps are there and intentionally created within this constructed environment.
In this image · No.6
Abuse
The newspaper and towel reflect the styling of Summer of 42, shown in No.40.