I teach K--she will be fine. If you have major concerns about her shyness or anxiety level, talk with the counselor ahead of time and see if you can get in touch with some parents and/or the teacher in advance. If this happened in my class, I would probably assign a particularly friendly student to be a "welcome buddy" to your child and help her get through the first few days.
Don't beat yourself up about this, it really does happen all the time. Your daughter might actually be the cool novelty new kid. One year, I had a new student come during the very last week of school. We had end-of-year wrap-up collages going, so I had to quickly assign her some brand new BFFs and had a volunteer take them out to the playground and snap some pictures, and then we took a new class photo in the classroom so she could be in it. The kids got into it and wanted to re-take previous pictures with the new girl so she would feel included. In K, the kids are really welcoming. She may be understandably anxious now, but rest assured that she will be fine once she gets there.