Decide which type of strength you want to communicate to the fifth-graders. You might decide on looking at the physical or emotional aspect or even the fortitude that comes from faith. Or, you might decide to communicate all of the various interpretations of strength and the ideas that they include.
Decide on what you feel the important aspects of strength are. Depending on the type you are teaching, you will have different aspects to talk about. You need to define what "strength" means to you before you can communicate it to your fifth-graders.
Create lessons that illustrate your specific ideas of strength, and why it is important. Depending on your definition, you will have different details that you'll want to teach. For instance, if you are teaching physical strength, you'll want to have lessons that talk about the areas of the body and how to make them strong. If you are talking about emotional strength, you'll want to discuss what this means, such as being comfortable in your emotions, being able to handle your emotions or dealing with problems that arise.
Model your idea of strength for the fifth-grade students. Students in fifth grade need you to model what you wish to teach them, because without this they might not be able to fully understand the concept.