I'm not entirely sure specifically as it could vary. But generally, a visual novel often involves a story with characters and plot developments presented in a visual and interactive way. 'Kodomo no Jikan' might center around stories related to childhood perhaps, with characters experiencing various events during that time of life.
Probably not. It depends on the content of the visual novel. If it contains complex themes or some mild forms of drama that might be difficult for very young children to understand, it may not be suitable for all ages.