What do you mean by a legal diploma, exactly? Homeschoolers usually just "award" themselves a diploma upon completing the high school graduation requirements for their state to their own satisfaction, then prove competency to prospective colleges through a combination of homework portfolio (for less important subjects) and nationally standardized test scores (for core subjects). An official, accredited diploma is largely superfluous, especially considering that you'd want your children to take the standardized tests anyway in hopes of getting additional college credit.
If you still want official diplomas for your children, you'll need to enroll them in an accredited school or portfolio review service that can verify they've completed all the high school graduation requirements.
If all you want is a legal graduation, then don't worry about it. Complete the work at home, and attend a graduation ceremony at one of the
homeschool groups in your area.