With little kids, we need to make them get used to langueges by listening it very often. Then, we identify the writing of the languages. That's the first step

You can ask them to repeat some new words. With this method, they will be more sensitive with langueges and help us learn the grammar, vocabulary more easily.
I know some books that can help your kids: The Value of Chinese Books for Beginners, Beginner Obstacles to Reading Chinese, http://appnaz.com/android/papers-please-com.appswiz.papersplease, Read Mandarin Chinese: 5 Great Books for Beginners