Wow, all I can think is how exceptionally lucky these kids are to be taught these concepts/exposed to these problems at such a young age by a willing and enthusiastic teacher. Most people professionally employed as K-12 math teachers wouldn’t even be able to teach this kind of curriculum well. I looked at the author’s blog history to find out if they were doing this for some huge amount of money and found a blog that suggests not only were they doing it for free (skimmed it, but didn’t see any reference to pay) but they had to try to convince other parents to let their kids participate! https://buttondown.email/j2kun/archive/a-foray-into-math-cir...

In my opinion, there is a relatively unknown (to those outside mathematics) huge “privilege” gap in mathematics education that makes it so those that only follow a cookie standard or accelerated curriculum are relatively unprepared for careers in mathematics compared to those tutored (or taught in special magnet programs, or by their mathematician parents) in these kinds of non-standard-curriculum concepts from a young age. Mostly, the problem is that the standard curriculum is almost purely rote-computational until you become a college ~Sophomore and it abruptly changes to being open ended and proof-based (which is the world most pro mathematicians live in) requiring skills in creatively applying logic. So students with this kind of exposure from a young age have a much easier transition to that while also scooping up all the math-career-builders like early papers and contest wins on the way.

Those other parents probably don’t know this but OP is providing an immensely valuable service that is hard to find in some areas and which some parents would pay a huge amount of money for.