If a game or software isn't designed for children, but children do indeed choose to play your game; does this project cover such a scenario?
Case in point: Minecraft wasn't specifically initially designed to be for children. The art style was due to artistic and programming engine limitations. One may consider that a checkbox for 'are you over 18' is not good enough, however, a number of adult users might not wish to subscribe to anything that may track them if credit card or phone number is required.