what's with this thing about putting little itty bitty extra parts on models?
It's called "Super-detailing" and separates the "model builder" from the "Advanced Modelers". and "Master Modelers".. All done in the name of making the most detailed and realistic-looking model one can, and one that will stand up to extreme close-up photography... "Customizing" cars is no different than doing conversions on or back-dating armor and aircraft...
Adding spark-plug wires, hoses, and wiring harnesses, seat belts, fabric interiors, along with other super-details is simply the "next level"...
While it's not aimed at kids and their first few kits, it's definately a thing that a kid can aim for after a building a couple dozen... Not to mention that plenty of "Advanced" modelers arre indeed, kids.. I started being a "serious" modeler and adding all kinds of extra details to my kits, as well as building diormas when I was 14...
As far as getting them started in hobby, that's kind of up to adults now, and it must be pointed out to them, as well as adult beginners that ,as with everything else in life, it's "Crawl, Walk, Run"...