Wrt imaginary numbers, just a thought: capacitance and inductance in an AC circuit.

I don't know what age group you're targeting but I found solving problems in this space gave me a great intuition for imaginary numbers; I found it quite interesting too. Heck, AC circuits are great real-world playgrounds for differential and integral equations - Laplace transforms too.

I've been fretting about how to help my kids get a better understanding of math for years. I've been bouncing between too simple and overly complicated for too long. :-(

I think I'm just going to mandate they spend some amount of time per week using Mathematica and going through some introductory book. At least they will taking baby steps in a direction better than youtube. Grumble...

So I've been fretting on which book. Any suggestions? Heh heh...