If you're teaching kids about light and darkness in the Bible, John 8:12 to 8:20 is a good place to start. These passages remind kids that Jesus is the Light of the World and that darkness will flee the Light. You can also remind kids that they reflect God's light and that they can use their light to drive away darkness.
Jesus is the Light
Read John 8:12 with the kids. Discuss the passage and reassure them that those who walk with Jesus will never have to walk alone or in darkness. Reinforce the idea that light drives away darkness by simply turning the light in the room off and then on. Use this as a metaphor for Christ's light. You can follow this lesson up with a craft. Draw templates for light switches and let the kids decorate one for their room. If you have the budget for it, consider buying plain white light switches and letting the kids paint them.
The Cookie Lesson
You can base a lesson on John: 20. The verse deals with the choice between darkness and light. Those who like to do bad things and sin choose not to be near the light. That's because if they come closer to Jesus, they will see the depth of their sin. Relate this to an experience that a child may have had. For example, ask if anyone has ever taken an extra cookie or candy even though they were told not to touch it. Not only were they disobeying, it was also a sin because it was stealing. Ask the kids to remember if they enjoyed their stolen item. Ask if they got caught. Point out that getting caught is not enjoyable. Sinners who don't follow Jesus may sin until they get caught and are shown the error in their ways.
Memories of Light
This lesson reminds children of the danger of choosing darkness over light. It's an activity that you can perform in Sunday school class or at home after dark. The child should take a flashlight and sit in a dark spot with all lights off. She should try to imagine the room and its details. As she sits, she should also think about the difficulty of remembering details. After a few minutes, she should try to remember the details again. Does it get harder to remember the longer she sits in darkness? When she turns on the flashlight, she should look around the room to see if things are different from her memory. Reinforce the lesson of the activity by pointing out that once you turn your light back on light, it gets easier with time to forget what the light has shown you. She should take the time to pray and ask God to shine his light through her.
Read Matthew 5:14-15. Stress to the kids how a lamp is not lit and then hidden under a bowl -- it should instead be placed high so it shines on everyone. Turn on a flashlight and shut off the overhead lights. Use a bowl or a thick towel to hide the light. Point out that the light is wasted when hidden and it's pointless to even turn on the flashlight if you're just going to smother the light. Relate this to times that the kids may want to hide their faith, worried that they'll be made fun of or disliked because of it. Remind them that Jesus taught that by shining their lights, his followers could do good deeds and reaffirm the faith of others.