One of the easiest and most interesting ways for kids to learn how to give presentations involving bar graphs, line graphs, and pictographs is using the topics that interest them the most. Assign surveys for them to do with friends, family members, and classmates about what their favorites are and make a tally as the answers are given. Then they can convert their responses to whatever type of graph is most appropriate for the situation.
Here are some examples of favorites that could be graphed by your students in elementary and middle school: favorite food, color, pet, teacher, t.v. show, video game, clothes, movie, actor, singer, song, grade, book, place to visit, hobby, sport, season, dessert, animal, car, toy, board game, card game, restaurant, store, etc.
There are many more that the students will think of which would be interesting to your particular class. The choice is not as important as the integration of visual aids into the mode that is used to present information, especially as they move into the upper elementary and middle school grades. Colorful graphs really add clarity and depth to both written and oral presentations. This helps the visual learner, whether he's the one presenting or the one receiving the information. Most kids take pride in making a great visual.