So much depends on the age and background knowledge of the kids. Also important is what exactly you want them to know. Are you going for a general "make food from the sun" or a very specific understanding of how the sugars are produced. The latter would probabaly not be developmentally appropriate (I say probabaly, becuuase there are always exceptions, but they are rare). However, you can always start building knowledge about the effects of the sun via sunbleaching construction paper, paper circles of different colors paperclipped onto leaves for a few days. The difference in the leaf color always starts questions.
You can build background knowledges of how the sun energy changes matter in many ways. Keeping with plants and leaves, you can do it with sun tea; one in full sun, one in the dark such as a closet.
You can begin to explore plant respiration by putting bags over leaves (just a couple) and watch the moisture inside build. For reference, you can tie a similar bag to a stick nearby so the students see the different between the bag with a leaf and a bag with nothing.
Hope this helps!