Former unschooling mama, here. I come in peace.
First, yes, it is very normal to have a non-reading 6 year old. Do not worry. They will accomodate, I am sure.
Second, as a former unschooling mama, I can relate to your feelings and I certainly empathize with your concerns. They are valid. It is hard to have a kid whom you believe is behind others, and is about to be thrust into a situation that may involve extensive reading.
Third, questioning your decisions, as well as the abilities and attitudes of your kids seems to go hand in hand with unschooling.
Do you read? I mean, of course you are capable, but do you read for enjoyment? Do you spend time sitting in quiet reading or reading aloud? These, we found, have been the key to getting the kids into reading. We had become so busy with life that reading wasn't even a daily thing for us (as so many people will atest is a very critical thing at this age). Once I realized how much they loved sitting with me to either hear me read, or to look at books ("read") beside me while I read to myself, I made it a point to do it as often as I had time to.
Finally, this is how it happened for DS (now 7.5 and reading Magic Treehouse chapter books):
While still unschooling, DS mentioned that some friends were good readers. I offered to help him learn. We tried the book, Teach Your Child to Read in 100 Easy Lessons, but we were not as consistent as we could have been. We borrowed Bob books from the library. He began reading the first level just as he was turning seven. He did not read often (not even every day), but slowly he began sounding out words everywhere we went. Little by little, DS used the sight words and what he had learned about CVC word families from the Bob books to sound out larger words. He is not particularly fluent, but he is now working on poems weekly to help with that part.
So, after some crazy months for our family, we made the decision to put the kids into a charter school. Thus, as we are no longer unschooling, I can't say what would have transpired this year in terms of his reading skills as an unschooler. However, when he entered the charter, he was considerably behind the schooled kids whom had two years of phonics crammed into them, as well as having memorized 120+ sight words. This has, however, not deterred him. One of the beauties of unschooling/homeschooling is that YOU are the primary influence on your child at a critical time in their development of self and self worth. We worked hard to instill in our kids that it is not about what they canNOT do, but about what they are able to do well, what they feel good about, and WHO they are that really matters in the grand scheme of things. And, for the most part, DS feels quite proud of his reading, regardless of the fact that his best friend at school is an amazing reader (reading at 7th grade level as a second grader). He now spends most of his spare time (by spare, I mean idle time not spent PLAYING, not just time he isn't working at schoolwork or at school), reading alone, reading to myself or DH, writing, or just playing school.
I am not sure if and how this post helps at all. Maybe my point is that kids will do what they see us doing. If you read, and read often and enjoy it, she will possibly do the same.
Good luck to you...