It's really easy. All you need is to crush your cereal in a bowl, get a magnet and stir it around. Pull it out and the iron in the cereal will be attracted to the magnet and the rest left in the bowl!
This experiment happened when my girls asked me the other day if eating iron could kill you. I told them that our blood is iron rich and we need it to keep us healthy. People who don't eat enough can become anaemic, which means they suffer with anaemia (A condition where the person has not produced enough red blood cells or haemoglobin in their blood, making the person suffer with weariness and their skin to pale.) So I showed them proof that it was in their food which they found most amusing and learnt a bit of science in the process.
I did say that too much of anything wasn't a good thing and eating all my iron filings probably wasn't the best thing to do and they were content with that.