This one is a little tricky. To my ear as a native speaker, "Every child should have access to a robust education" sounds correct.
You probably mean "access" here as an uncountable noun. That is, you aren't talking about one access versus two accesses, but rather about access as a general idea. So it does not take an article. "Every child should have access ..."
"Robust education" is more debatable. My intuition is that each child is getting one education, the education of that child, and not accessing education in general. Thus I'd say "to a robust education". But it would be equally arguable that children are getting access to education in general, so "to robust education" is also correct.
I struggle a bit saying why it is general access but a specific education. I think the answer to that is that a child could have many attempts to access education spread over a period of time. But when he's done, he only got one education.