I think most children are interested in comic story books. Monkey King Chinese,which I would like to share with you, is a series of elementary Chinese language primers for primary school children from the 1st year to the 3rd year in English-speaking countries.
Search it on amazon to buy it, and...
I think it is better for your kids to learn Chinese online. It is a great choice if you are not in China. Native Chinese teachers are easy available on internet, and they are always there if you need.