A precise but simple explanation of the difference between a mixture and a solution is that a mixture is a substance made of other substances that are mixed but not joined together, while a solution is a substance made of two or more substances that do join together. If the kids are older, it should be noted that a chemical reaction takes place to create a solution and that no such reaction occurs in a mixture.
Explain the difference between mixtures and solutions to kids in basic terms they can understand. Then, bring the explanation to life by providing concrete examples. You can even have the kids experiment with their own examples, so they more fully remember the concepts.
To bring these concepts to life, the kids can be asked to create a mixture of toys or pieces of candy. You can then move on to mixtures that may be more difficult to understand, such as olive oil and water. When mixed together vigorously, these substances seem to join together. For this example, let the container of olive oil and water sit for a moment so the kids can see the two substances separate and can more clearly understand the substance as a mixture. Then, let them try the same thing with salt and water, so they can see how, in a solution, a solid dissolves into a liquid and the substances are joined.