S.L.
As a K teacher (as well as parent of a K student) I'd say they can learn a lot of reading, writing, addition, subtraction, etc when they get to K IF they are ready to learn. Most do know their letters, shapes and numbers when they arrive but more important that they are ready to learn, which means sitting still listening, taking turns, following directions. focusing on their work,sharing things with peers. I never homeschooled and always thought the hardest part of homeschooling would be Pre K and K at home as it is so much about adapting to being in a group of many children, sharing, not calling out, taking turns, being more independent. It is expected they are taking care of zippers, buttons, bathroom needs and are able and willing to communicate their needs to the teacher ie "I need a new pencil , I don't feel well, I need to use the bathroom now, not in ten minutes," etc. and that they can separate from Mommy without a Long crying jag.
You can help prepare them by reading to them and talking a lot about the stories, predicting, comparing, giving opinions, relating the story to own life or to another book. You can play games that require following directions, taking turns and losing and winning gracefully. Also attending library story times and other activities where there are other children would be great.