When I get asked how I know what to teach my children my common response is: I don't teach them; I help them to learn. That means giving them the time and space to explore and discover on their own, but it also means recognizing interests and abilities and learning styles and identifying resources to help them to learn.
For math, helping children learn may mean investigating various math curricula, using an online math program (I like IXL.com), and/or tapping into ample community resources.
One of the great benefits of homeschooling, I think, is the opportunity for our children to learn subjects from subject-matter experts: people in the community who are passionate and knowledgeable about their field of expertise. Children learn history from historians, botany from botanists, economics from economists, science from scientists, art from artists, and so on. For city homeschoolers, these subject-matter experts are everywhere--at museums, universities, libraries, community centers and enrichment programs, and in our own neighborhoods--and they are usually very eager to share their wisdom with others. For example, our neighbor is a biology professor specializing in snails and mollusks, which my kids happen to be very interested in, and he provides information and resources that ignite their imagination. Dropping off snail shells, letting us "babysit" his lab's hermit crabs, talking with the kids about some of the vocabulary they have learned from their books (what is the correct way to pronounce "operculum" and "anemone"), has led the children to want to learn more, to read more, to know more.
The same is true for math. Homeschooling allows our children to learn math from actual mathematicians--those who have studied and trained to be mathematicians, who are currently practicing mathematicians, and who are passionate about getting others excited about their subject. My kindergartner has developed a strong interest in math, especially multiplication, and I thought I would see what community resources might be available to her this fall. Through my homeschooling network, I discovered a local, MIT-trained mathematician who teaches math workshops for homeschoolers and non-homeschoolers and is adored by those who learn from her.
I wonder how many of us would be more excited about math, less intimidated about teaching and learning it, if we learned from a passionate, practicing, trained mathematician when we were young.