I use www.raz-kids.com with my students. I attach these letters in addition to the login cards and parent letter that are available on the teacher section. I have a systematic approach to how I would like my students to use this program and these letters describe items like only take a quiz up to 2 times etc.
Kindergarten registration packet: Build a culture of fun family learning activities for school success. Ideas in these parent letters provide ABC phonics and handwriting support. Accelerate kindergarten readiness when students start school knowing the “ABC Phonics” fingerspelling song! Use for a strong parent partnership ESPECIALLY in a Title 1 kindergarten. | parent letters + fun family activities + free printables + ideas http://nellieedge.com/weekly-focus/spring-kindergarten-registration/