Fourth graders can understand the idea that marriage is a sacred promise made between two people to love only each other for the rest of their lives. A child doesn't need to comprehend the physical act of adultery to understand the concept of fidelity. Ten-year-olds have both broken promises to others and had people break promises to them. Discuss how it feels when people break their promises. Stress that marriage is a really big promise that's easier to keep when you ask God for help.
If you have a child from a broken home in your class, be sensitive to his feelings when teaching this commandment. A child from a broken home may already have anger toward one parent. Walk a fine line between condemning the sin of adultery without condemning a sinner. Don't put the child on the spot by asking him how a parent's broken promise has affected his family. Keep the discussion focused on broken promises in general, not on specific family situations.
When teaching the 10 commandments, it isn't enough just to focus on the "Thou shalt nots." Children need to learn how to keep promises as well as how not to break them. Explain that God does not ever break His promises. People, however, will always struggle to do the right thing. Discuss the importance of asking God for helping in keeping promises. Stress that unlike people, God does not change and will never change. He can always be trusted.
When you teach the seventh commandment, make two points clear to children. One is that marriage represents a promise to God as well as to another person. The other is that even when you try to do the right thing, it's easy to fall into temptation unless you ask God for help. Use the story of Joseph and Potiphar's wife from Genesis to emphasize that it's difficult to stay morally pure, but it has great rewards.