Leading Little Ones to God, by Marian M. Schoolland, has been around for 45 years and is one of our favorite resources for explaining biblical doctrine to our kids. Divided into 14 sections and 86 chapters, it systematically teaches children biblical theology in bite-sized chunks without using weighty seminary words. Children will learn about the doctrines of sotieriology (salvation), harmartiology (sin), ecclesiology (the church), sanctification (spiritual growth), eschatology (future things), among others. We first used this book when our eldest child was in kindergarten and find it engaging for toddlers through about 3rd grade.
It is my experience that children understand a whole lot more than we give them credit for, that they can engage in much more intelligent conversation than we allow them to, and that they need to know the true God of the Bible in His majesty, fury, and amazing love not the one of the cartoon coloring page we give them in Sunday School. Working your way through Leading Little Ones to God is a wonderful way to introduce your children to this glorious God.