Start with teaching the first graders how to express their opinions in writing. Give the children a topic to write about, such as their favorite food, the best cartoon on television, or the best kind of pet. These topics are simple enough that the children will know about them, and they are all topics that most first graders will have an opinion about. They can then share the opinions and discuss who in the class agrees or disagrees, which will aid their understanding of persuasion.
After children have a solid understanding of persuasion and the reality that other children may have different opinions, you can introduce the idea of persuasion. Ask the children to write another three sentences to convince their classmates who disagreed with their opinion to change their minds. For example, a child who wrote that dogs are the best pets can try to convince the children who preferred cats to change their minds. Alternately, the children can write a persuasive piece about a topic in their community, such as recycling.
First grade is the time to introduce the concept of audience. Ask the children what their parents might say, for example, if they asked for a puppy. Then have the children brainstorm ideas for convincing their parents that purchasing a pet hamster is a good idea. Talk about strategies for convincing the parents, and then how these strategies might change if they were trying to convince their teacher to purchase a hamster for the classroom.
When you teach writing to first graders, do not set your expectations too high in terms of content or the quality of the writing. First graders are capable of understanding audience and persuasion, but their logic may not be well-developed. Their reasons for persuading their parents to get a dog might be "because I want one," for example. Expect their writing skills to be basic, as well. First graders often have misspelled words, sentence fragments, punctuation errors and word order problems, for example.