Before you start worrying about an expository topic for third grade students you need to try and understand what it means to these students to write some of these essays. First of all, we must understand the fact that at this level the students basically do not know a thing about whatever it is they are supposed to write. For the same reason therefore it makes sense for you to ensure that by the time you are working on this, you have all it takes to deliver a strong argument, and at the same time give them something to think about.
Since the kids are just developing skills such as critical analysis, you should not spend a lot of time asking them to write things that they do not understand. Take them slow and work with topics that are easily understandable to them. This way it will be a lot easier for them to write the task, and for you to read and understand what they have written. This makes it easier for you to help them in your capacity as a teacher, parent or guardian. The following are some topics that you can start them off with:
The titles mentioned above are just but some simple ones that you can use to help the individual learn how to write some simple topics. What we also realize from these is that they work closely along topic lines where the student is comfortable.
In their age group, it is important to make sure that you get the student to write about things that they see on a daily basis, things that they can relate to. This makes it easier for them to reason and put their ideas into context. If you can do this with them, they will easily learn to write good papers in the future.