I vividly remember cramming for a test my freshman year of college, not having things click, and the final Aha! when a semester of pain disappeared with a few visualizations. The contrast between how most classes presented the material and what actually worked for me was jarring, and I had to share what helped. I hope other people share what works for them, in any format they can.
tx kjhughes
EDIT: I'm working through Khan Academy with one of my kids...would like to find other resources as well.
Look for opportunities to expand what they're doing in school. For example, after my kids had covered place values in school, I taught them binary. Their knowledge of base 10 is so much more robust when they learn base 2 as well.
Point is, I bet you already know plenty to be that ideal resource you seek for them. Good for you for wanting to start early.
If I may similarly influence anyone here, for themselves or someone they know, to read Calculus made easy to supplement their calculus coursework, I will be happy to have paid the favor forward in some small way.
By the way, Kalid Azad may be our modern day Silvanus P Thompson. And he has better tools[1], which he wields masterfully, than just pen and paper. Recommended too.
[1] https://betterexplained.com/