By the time your child is ready for kindergarten or school, they should be able to recognize their own name and other simple written words. The sounds of each letter of the alphabet should be familiar to your child, and they should understand the principle of reading from left to right, which way to hold a book, and possibly even be starting to read three and four-letter words.
Since 3rd grade math relies on the concepts that were learned during kindergarten, first and second grades, don't be afraid to start your child at a lower level.
This sounds like an awful lot, doesn't it! A good set of preschool worksheets should cover all of this, and more. In the preschool years, repetition is the key to learning, but you should look for worksheets that teach the same skills in a variety of ways. This not only prevents boredom setting in, but also reinforces the concepts by encouraging understanding as well. The importance of reading to your child cannot be emphasised enough, and you should encourage them to 'read' as much as possible too.
A K-12 Digital education must provide creativity; customized learning based on the needs of the student, and cutting edge digital educational materials for grades kindergarten through 12th.