I taught third grade for two years and teach fourth now. Keep in mind that they are about a fouth of the way in the year and, depending on the area you are, some may still be at a second grade level as far as ability goes. I would expect that each child will be able to come up with a topic sentence and four supporting details to support a paragraph for sure. However, I have successfully taught five paragraph essays in third. A good topic is popcorn. You can use a web design to have a branch for taste, texture, smell, etc... Each obviously gets it's own paragaph when you write. Model, model, model is KEY!! (PM me if you want more help...I could go forever!
)