Unfortunately if you follow the link to the proposed site for publishing kid-friendly mathematics articles, then search with a filter for mathematics, you get zero results :(

fwiw I have made some attempts to get my own children interested in 'advanced' mathematics. I found the TV series "Story of Maths" by Marcus du Sautoy to be helpful. I've also had some positive results watching lectures from Harvard's E-222 class[2] with my older son (14).

[1] https://www.netflix.com/title/80093836 [2] http://matterhorn.dce.harvard.edu/engage/ui/index.html#/1999...