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Practice.

Don't just read, binge YouTube, or click through banal tutorials with a misplaced hope of learning through osmosis. You learn math by doing math. Most likely without a computer. It's unpleasant as you will be quite bad at it, but that's a good thing as it means you can get better.

I'm a bit younger but also with kids and have been using the fracture in life that brings to fill in similar gaps in education. UoL have some courses as part of an online degree. I don't recommend it, but the references are good for fundamentals. I've been collecting them and some study notes here: https://simple.industries/notes/uol-comp-sci.html.

The Susan Rigetti guide (https://www.susanrigetti.com/math) already posted is also an incredible reference.

Also, learn with your kid(s)! There a whole set of publications around the math circles movement. I'm currently working through this with my 3yo: https://www.goodreads.com/en/book/show/12689040.