The unbelieving child needs, before anything else, to be regenerated. With this child the acceptance of the gospel is priority. In each lesson give both a clear presentation of the gospel and an opportunity to respond.
NOTE: Don't think that time spent presenting the gospel is wasted on the believing child. With each time he hears, he will come to understand it better as well as get to know how to communicate it to others.
The believer knows Christ as his Savior and now needs you to teach him something he can grow by. Pick a simple truth that can be brought out in the lesson that relates to spiritual growth.
Example: Tell others about Jesus.
Remember to tell them how they can put it to practice and challenge them to do so.