Activities to develop math skills and growth mindset in math - the founder, Jo Boaler, makes a strong case against the idea that some people are "math people" while others are not. The site includes lesson ideas, tasks, and research articles, including some directed specifically to parents.
The National Council of Teachers of Mathematics (NCTM) has a website called Illuminations, with online lessons and activities. This logo links to brain teasers and puzzles, but you can explore more on the site, including a Webcast Library. The brain teasers are not visually inviting, but they are mathematically rich.