Free Comprehensive Math Curriculum
Over the past several decades, there's been a lot of grant money going to math advancement in the United States and England, and the result of some of this grant money is some really solid, comprehensive and complete, curricular materials that are available absolutely free.
​
Note that although there are a number of vintage math textbooks available in the public domain, some of which I rather like for supplementary use, I don't recommend using them as your primary curriculum. Modern curricular materials introduce geometry, algebraic thinking, and logical thinking much earlier, and research shows that this really helps students to understand more advanced math down the road.
Free Math Curriculum for Elementary, Middle, and High School
My three favorite free math curriculum all go from Kindergarten through high school.
The Mathematics Enhancement Programme, honestly, doesn't look like a lot out of the gate: the website is very old and kinda messy. But this is a SOLID math program. I used it for a few years with my oldest, and continued to use some aspects of it even when I taught in a classroom. Note that this is a UK program, and students start "Reception" when they are 4. You may want to use the Reception year as a preschool program, or start it in Kindergarten before moving on to Year One.
Open Up Resources is a nonprofit that produces free, open-source math and ELA curricular materials. Everything is free, although you do need to create a login to see the materials.
Illustrative Mathematics is another nonprofit organization that provides free math curriculum for students from kindergarten through 12th grade. It was developed at the University of Arizona.