Start teaching Czech reading and writing right away!
My older sisters and I were all taught to read and write Russian at home at the same time as we started going to school and learning English reading.
But by the time my little sister came along my mom had a full time job and couldn't dedicate as much time to teaching her to read and write. She knows how, but not nearly as well as my older sisters and I, because it wasn't as strong in her schooling.
My mother would read Russian books to us from birth on, so we learned to distinguish Russian characters from English characters even before we could read ourselves.
Some people say learning 2 languages at the same time (speaking or reading/writing) can hinder development but I completely disagree. The fact that I learned to read Russian at the same time I learned to read English means I am equally capable in both.