Plan something, let's say for Palm Sunday or Easter Sunday when you know most people will come and bring their children. Put out a notice in the church bulletin or send postcards directly to the child's home announcing your plans for this fun class. Make sure this is a day when the lesson is full of activities, and things the children have made and can bring home. Tell the children you are starting an Attendance chart and if they have perfect attendance they will receive an award certificate. At our church they are very good about letting the teachers have time on special Sundays to announce and pass out special awards.
Children love getting mail.
It is a small amount of extra work and time with big results. It has helped me keep the children in class on a regular basis. I recently did a lesson on the Armor of God using the materials below. I had the patterns copied onto poster board and had the children color the shapes and we made a hanging mobile out of them. I also didn't hand out all the armor at once, as we read the scripture the pieces were passed out as we got to the piece of armor and the kids gave examples of how they should use that armor today in their lives.