The steam page features this in the description:
This game is not suitable for children
or those who are easily disturbed.
It constantly reminds you both in and out of the game that the story isn't suitable for children of easily disturbed audiences.
The visual novel of DDLC is developed on the Ren'Py engine, which my visual novel is being coded on. This all the more
Personally, I was impressed with the use of transitional effects, from screen tears to pulsing veins that pop up on the screen in the viewpoint of the player. The author of DDLC granted permission for other Ren'Py users to use his coding for transitions and effects. This immediately led me to experiment and play around with their uses.

DDLC Homepage on Steam:
Team Salvato. (2017). Doki Doki Literature Club. Available from: https://store.steampowered.com/app/698780/Doki_Doki_Literature_Club/ [Accessed: 3rd October 2017]