You hire out someone who can, or send the kids to a community college.
There’s also Saxon Math, probably the best math curriculum I’ve ever seen.
I used it with my kids and earned a lot as I went. I’d have to read the prepare the lessons ahead of time and because of how they cover and explain the material, I finally learned all that math that the public schools made so confusing and after decades of thinking I was bad in math, found out I was actually good at it.
They also have math from K - 12 but the place to start is with math 54, fourth or fifth grade. You can find much cheaper options for teaching the basics of skip counting,
addition, subtraction, multiplication, and division.
Excellent solution. Once I retired, I got very bored. Unusual circumstances put me into a local high school teaching Embedded Computing under a provisional certificate. (I had to get a certificate within three years.) I really enjoyed teaching; but, I saw the actual causes of public failure: Students and parents.
My class was well outside of the box for students. In a school of 3600 students, 25 joined the class. No matter how hard I tried, about 10 scored 95 or above and everyone else below 30. Students either engaged, or didn't. Parents of the low scored students often berated me for their child's low scores, even when I showed them how many were knocking out 100s.
I taught for three years. The school gave me everything I wanted anytime I asked. That amounted to over $30,000 worth of lab gear. Students routinely told me that the class was their most favored in all of high school; and, many selected college courses and career based on that class.
Getting a teaching certificate at the ripe old age of 72 seemed pointless, so I left after that three years. They were confident that they could continue the class; but, it turns out that deep subject knowledge was indeed necessary.
I tried to find private schools who would want such a course, but was ghosted following any attempt at contacting them.
