Start with a story. One of the best ways to communicate with a small child is to help him relate to the topic through a simple narrative. A book about car safety--perhaps with a title character who is learning to buckle his safety belt every time he gets into the car--is a great way to start. Some good examples are "Do Kangaroos Wear Seatbelts?" by Jane Kurtz or "On The Road" by Claire Llewellyn.
Read the story aloud with the child. You'll want to then open a discussion about what you just read. Assess whether the child comprehended the message of the story; encourage her to summarize the plot back to you.
Open a box of crayons.
Drawing is an effective method of communication for preschool-aged children. Children often find it easier to draw feelings since they can't always articulate them. Ask the child to draw a picture of his favorite part of the book. When he is finished, ask the child to describe what he drew and why.
Supply the child with a coloring page containing a picture of a child strapped into a safety belt. Coloring an image is a way to help the child remember the message.
Practice buckling. Take a little trip out to the car. Demonstrate how to secure and free the seat belt safety latch to the child. Let her then practice the process as long as she pleases. Ask her to demonstrate to you or someone else how to work the belt.
Sing a song. Young children often remember material more efficiently when the information is put into the form of a melody. Come up with your main message, and then put it to a familiar tune. For example, sing the words "When you ride in a car. You will be a star. Buckle yourself in. Everyone will grin..." (or something like that) to the tune of "Three Blind Mice." The song will get your point across while making the lesson fun to learn.