"How will they learn everything they need to know?"
Do the best of the high school graduates know everything they need to know? No, and at some point, ideally, they start learning on their own. Some fail to get to that point, though. Unschooled kids have a head start. They know how to find what they need to know, and they have not been trained to ignore things that won't be on the test.
When parents see how and what their children are actually learning instead of just scanning for the half dozen school-things, unschooling will make sense to the parents. If you wait for school to congeal from a busy life, you'll keep being disappointed. If you learn to see everything instead of just school things, unschooling will start working for you. When you see it you will believe it.
SandraDodd.com/seeingit
photo by Ashlee Junker