Focus first on your relationship with your child. Try to slow down and enjoy your time together. Talk about math when it comes up, but if it's not interesting or enjoyable, don't push it. The best learning takes place when children feel safe, loved, and respected.
Some children love baking. Others love playing board games or sports. Some enjoy trying to do mental calculations quickly: others freeze up when put on the spot. Notice what your child is curious about and enjoys: start there!
Children are curious and creative when they're engaged in something they care about. If you ask them questions about what you're doing together and invite them to help you solve problems, you'll be amazed at what they can do! Children learn through doing and talking, so make sure to leave space for them to take charge, within the realm of what's developmentally appropriate of course.
There are always different ways to think about or solve a mathematical problem, and there are many ways to make sense of any mathematical idea. Ask your child about their thinking. Share your own thinking. Just don't suggest that there's only one way to approach the problem or idea. The ability to think about problems and ideas in different ways — and the ability to make sense of others' thinking — is a sign of strong mathematical understanding!
Children need time and different experiences to learn deeply. Don't be alarmed if your child doesn't recall learning something at school: they might not recognize the same idea or concept in a different setting. They also might not have learned it yet, because schools and classrooms introduce topics at different times, and that's OK. Focus on talking, noticing connections, and making sense of ideas in context: this will help your child attach meaning to the mathematics. When children understand the mathematics deeply, they are better able to remember it over time and apply it to new situations.
I read about math a lot, and I love the following books. The last one addresses teaching and learning. The rest are simply about the beauty and marvel of mathematics itself and are written for a general audience. It was tough, but I've put them in order by my favorites from top to bottom. If you can, please consider purchasing books from your local, independent bookseller.
Mathematics for Human Flourishing by Frances Su
The Joy of X: A Guided Tour of Math, from One to Infinity by Steven Strogatz
How to Bake Pi: An Edible Exploration of the Mathematics of Mathematics by Eugenia Cheng
Here's Looking at Euclid: A Surprising Excursion Through the Astonishing World of Math by Alex Bellos
How Not to Be Wrong: The Power of Mathematical Thinking by Jordan Ellenberg
What's Math Got to Do With It?: How Teachers and Parents Can Transform Mathematics Learning and Inspire Success by Jo Boaler