Absolutely. Key for me was to invite children's friends (and family) along, host it in our house and make it a recurring weekly thing. The books (presumably you also have these from the MSRI's 'Mathematical Circles Library') are great, but week-to-week I've found the free online NRICH resources much more directly useful: https://nrich.maths.org/about-nrich
It's been really rewarding. I definitely recommend jumping in. I started with my reception-age child (+ school friends), and have just extended it to their younger sibling (+ friends from nursery). Your 3.5 year old will have started the EYFS (Early Years Foundational Stage) syllabus at nursery if they attend (which is also what they do in the 'Reception' year at primary school, before starting the national curriculum in the first year), so they will now be exposed to counting and comparisons. The perfect time to get started, in other words!
There's some NRICH funded research that showed that exposure to symmetry and reasoning at this level was much more predictive of future abilities than numbers and counting. I think when parents try and help at the early stages, they often try to e.g. get their kids to count to 100, which is conceptually identical to counting to 10.
For number fluency there is the free White Rose '1 minute maths' app, which does a very nice job of gamifying subitising & etc. A lot of primary schools in London seem to have adopted the White Rose teaching resources.
https://whiteroseeducation.com/1-minute-maths
Practically speaking (running a little maths circle in the UK for my children, and some of their friends from nursery and primary school), I have found the Nrich website to be the single best source of resources:
https://nrich.maths.org/about-nrich
The book by Zvonkin described in the article is a very good motivator, particularly for the honest descriptions of lessons gone badly wrong, and staying up late cutting out pieces of cardboard! But it's quite difficult to use as a teaching resource.
What do you not like about Feynman's "little arrows" / rotating clock hands in the QED book? I can't think of a more simple metaphor for the exponential of a complex phase, exp(i omega t). I suppose you could try and do it with more commonplace trigonometric functions, but then you lose the simple vector interpretation of adding the contributions. Or are you arguing that you should always try and teach complex numbers and the Euler identity to avoid strained analogies?
That's great! Their motivation & updates seem very sensible.
Calculus Made Easy is a very nice compact book for university students who have started to forgot their high school training, and I will certainly point people in the direction of this project.
This is super useful to know about!
The sprite designer & waveform editor / tracker is a really good creative introduction to computers for small children. And you can jump straight in to doing this with the above web link.
(For those new to Pico-8, hit 'esc' from the Lua console to bring up the editor tools, then click on the icon in the top right.)
But it is theatre.
A single £0.01 Zener diode can generate vastly more guaranteed (by quantum mechanics!) randomness, without all the possibility of failure, entropy leaking etc.
The speed of sound does not depend on pressure, only temperature.
I suspect this is occurring because the (sonic) flow through the nozzle cools as it expands, therefore the speed of sound drops, making the same flow now supersonic in the cooled gas.