One-on-One learning like what you're describing is kind of the holy grail of all education, or the second-holy grail after finding ways of making students interested in things they don't yet know/understand (which is also the case most of the time in the parent-child bond, because kids are interested in whatever daddy and mommy do). This kind of enviroment takes the child from being a helpless ape to a functional human with intricate understanding of very complex rule sets (language, society, technology,...) in about 15 years.
But learning enviroments are typically the more inferior Many-To-One variant, so even if your child isn't confused by RPN, and even if (n-1)/n children aren't confused by RPN, the teacher must clarify it nonetheless for the 1/n student who is (or appears to be) confused, a book writer might have even less options and more constraints.
Basically, education is a very complex human-to-human Serialization-Deserialization problem. Knowledge isn't what's in the books, or the video lectures, or on github, all of those are just on-disk "dead" representations, the result of calling toString() on actual knowledge. Knowledge is all the intricate in-memory data structures built up in all the human brains that are currently living. The problem of teaching is : given a brain, reconstruct some of its internal data structures in another brain with an error not exceeding some threshold.
It's bad enough that you have to serialize the very rich and graph-like patterns in your brain into flat streams (words, pictures,sounds) to transmit it, it gets much worse when you have to do it for multiple brains simultaneously, because each brain parses streams differently. The more brains and\or the less you know about each brain, the worse your teaching performance and the uglier the representations you come up with to transmit your knowledge.
This isn't to say that teaching is easy when it's parent->child, just that (assuming reasonably healthy bond) it's as good as it ever gets. A single receiver who is extremely interested in whatever you have to say and will re-try on error a lot of times to understand it, that's the teaching problem on easy mode.