You can use illustrations and graphics to great effect in teaching main ideas to kindergarteners. For example, provide students with magazines and ask them to cut out a picture they like. Once the students have cut out their pictures, have them draw their own, simpler, version of the same picture. Having students simplify the picture by reducing it to simple elements will help them provide the picture with a one-sentence title that captures the main idea.
Most kindergarteners have only rudimentary writing skills. However, this fact can be used as an advantage in teaching main ideas. Since kindergarteners can only write the most basic of sentences, if you challenge your kindergarten students to write a summary of a passage in one sentence, it will force them to write the central ideas, since they will not be able to write long sentences that describe intricate plot details. Of course, this strategy doesn't guarantee that your kindergarten students will understand the main ideas of a passage, but it can definitely make them think in that direction.
Most kindergarten students will have a hard time intuitively "getting" the main ideas of a passage, because kindergarteners are typically too young to have abstract thinking skills. However, most kindergarten students will be able to identify the supporting facts in a passage, since supporting details are basic and easy to understand. One strategy you can use to teach main ideas to kindergarteners is to give them a sheet in which one box is attached to three other boxes. Once your students have the sheet, you ask them to fill in the three peripheral boxes with supporting details. After they have completed that task, you ask them to think about what the main idea could be, based on the details used by the author.
Kindergarteners are not too young to benefit from being given a quiz. To the contrary, a simple multiple-choice quiz can reinforce these students' understanding of a book's main idea. Here, the strategy is a long-term one. First assign a short book, such as a 5- to 10-page picture book for the students to read. Then explain the main idea of the book to the students. A few days later, give the students a quiz in which you ask them what the main idea is and which ideas support this main idea. Of course, you need to give these answers in class beforehand, since you can't expect them to figure these things out as they go.