At first I was thinking #1 as all mine are under 9, Now I am thinking of #4 because it should really all start with me and go from there. Then I thought well there is a reason they are numbered!
Volume 4, Ourselves, Charlotte wrote for the children to help them understand how reason, will, and character work and to encourage them in their personal development. If you’re looking for a practical and foundational first read, I would recommend either Volume 1 or 6.
I like to recommend reading the ones with “Education” in the title first: #1-Home Education (if you have children younger than 9), #3-School Education, #6-Philosophy of Education.