How should PhD student show importance of research proposal in cover letter requesting access to important dataset?
Your advisor's critique is exactly right.  You've made a convincing case that your subject is important, but not that your work is important.  This:


  My current research interest is concerned with the interactions between families, students, teachers and staff in Head Start programs


is the sort of vague writing that a lot of academics are prone to (I'm always trying to beat it out of my writing).  Being "concerned with the interactions" is a very abstract, high level statement that doesn't actually say anything.  Concerned with how?  Which interactions?

Finding out whether teachers say "hello" or "hi" more often to family members is an investigation of the interactions among the people you describe, but it's an utterly uninteresting one.

Start concrete, with the specific research questions you plan to ask.  Think about what you excited about your research. How would you describe your work to a non-academic? What's the story you that keeps you excited about how your research will actually matter - the line of causation from the work you're going to do to actually making people's lives better?