See that's the problem. These western rules of composition were developed over a period of time by painters who used all sorts of compositional "tricks" (forced perspective, multiple perspectives, multiple vanishing points, etc.) to generate the final image. Photography (as a single exposure image) doesn't have all that latitude to force a composition to work. So while your admiring the "triangular compositions" within the "Last Supper" that "make it work," you're missing the other more subtle compositional elements used within the image that reinforce the main compositional rule elements that you can identify.
Then if you study something like Chinese art history, you'll find out that their "rules of composition" are totally different than western (European) rules of composition. So, now just which rules are we to follow?
"The more you become familar with the cpompositional guides (not rules, ok) handed down from the Great Masters of the past through out Art history. The more successful your creation becomes. This may ensure your hard work, will be admired by more viewers."
Oh, give me a break. You don't really believe that do you? You mean, let's make rule bound photos because that's what the majority of people can relate to? Wow, that sounds like a lot of fun and a sure-fire road to interesting photos.
Sorry, can't buy into that one. There are only two types of photographs: the interesting kind and the boring kind. Slavish adherance to rules WON'T move a boring photo in the interesting category just because it follows a rule of composition. It will still be a boring photo composed according to a rule of composition.
Also, don't have the faintest idead what your attempted "children / adult" metaphor has to do with photographs.