A very common expository writing assignment is to write about a recent experience the child has had. The classic "What I Did During Summer Vacation" essay is an easy example, but you don't have to stop there. First graders can write about what they learned in class today, what their home life is like, a meal they ate recently, an animal they saw or their experience of a recent class trip. Be sure to pick an experience everyone in the class has had.
First grade students are highly relational and spend their lives surrounded by people: parents, classmates, teachers, friends and siblings. Assigning students to write a paragraph describing a person they know well can help them practice observation skills and teach them how to put people's physical characteristics and personalities into words. Emphasize the need to differentiate their subject from other people. It is not enough for a child to write that her father has a nose and two eyes. Instead, encourage her to write what color his eyes are or what makes his nose different from her own.
Many people enjoy discussing the things they love in life, and first graders are no exception. Children are likely to enjoy an expository writing assignment that allows them to pick their favorite in a specific category and then describe what it is and why they like it. Categories could include foods, pets, animals, toys or places. Give specific guidance to help students understand what they should describe about their favorites. For example, in writing about favorite places you may ask them to mention special memories about the places they choose.
A slightly more advanced form of expository writing is to have each child choose and defend one side of a simple debate or choose one option out of several and describe why he chose it. Be sure to keep the choices relatively simple and uncontroversial, such as what meal the child would serve a special guest or whether day or night is a better time for school. Defending even simple choices like these can help children develop a critical self-awareness about their own thinking processes.