You can first show students some paper clips and ask them if they think they will be attracted to the magnet. After making their hypothesis you can then let them try this and they will discover they are.
You then can ask students if they think that the cereals they have will be attracted to the magnet and have them give their reasoning. They can test three different kinds of cereal with varying amounts of iron in them. Total has 100% of the daily value of iron and will work the best.
They need to crunch up the cereals on separate plates and can run a magnet underneath the paper plates. They will notice that total is magnetic and other cereals are not as much.
They then need to read the nutritional facts to see if they can figure out why Total would be more attracted to the magnets than the other cereals. They will discover that it is the iron content added to the cereals.
Other ideas and extensions:
You can soak the cereal in water and then blend it. When you put the magnet in the cereal all of the iron pieces will come off.
Students can make designs on their paper plates such as mazes and can move the iron pieces from the cereal through them by dragging the magnet on the underside. They also can see how much iron they can collect in a certain time frame.