I've seen an example that works: The Danish film and video game rating system.
It differs from e.g. the US system in a number of ways:
* It's run by an independent government-sponsored organization, not the industry.
* For children not accompanied by an adult, the highest rating is "15 and older".
* Children ages 7 and up can see any movie if accompanied by an adult, no matter the rating.
* The board is charged only with determining if a film could be psychologically damaging to a typical child. They do not judge the "morals" and message of the film.
* The board features actual child development experts. As such, they know that cursing and nudity is not harmful to children, and if that's all the film contains, it will be rated "All audiences".
Example: "Harold & Kumar Go to White Castle".
USA (MPAA): 17+ (unless accompanied by an adult) due to "strong language, sexual content, drug use and some crude humor".
Denmark: 7+ recommended (but all ages admitted) due "strange and threatening persons, assaults, fights and accidents [...] all in a comedic context" (a context which could be lost on very young children).
To quote the ratings board:
The Media Council classifies films based on a perspective purely concerning harmfulness. The classification decision shall be made on the basis of an assessment of whether a film is considered harmful for children in that particular age group. When classifying films, we look at film effects, depictions of grievous loss, degree of realism, possibility of identification, inclusion of redemption within the course, genre and the expected media competences of the age group in question.
The Media Council’s view on child protection is that
* Children can manage a good thrill.
* Children are not likely to fall to pieces by the slightest push.
* Children are active users of media and, therefore, already in an early age, they have accumulated both media competencies and experiences.
* Media are good resources in children’s everyday life.
* It is acceptable that films frighten, though, only to a certain limit. The Media Council sets these limits.