An easy way to start with magnets is to give a child a couple of them and let her play with them. As she does, ask her what she notices. See whether the magnets attract each other, push each other away or do both. Determine why the behavior occurs and whether it is always the same sides that attract and push away or whether they change. From here, you can start talking about polarity in magnets and how opposite poles attract each other.
Ask your child what things he thinks magnets will be attracted to. If he's having trouble thinking of things, suggest some, such as a wooden table. After he has a list of what he thinks will attract a magnet and what he thinks won't, have him test out his theory. As he discovers whether he's right or wrong, talk about what those objects the magnet sticks to have in common with each other. From there, you can talk about how it's only objects with iron in them that are magnetic.
It's possible to pull iron filings from the dirt using a magnet. First, place a magnet into a plastic sandwich bag and then seal the bag to keep the magnet free of dirt. Then hold the magnet above a pile of loose dirt. If there is iron present, it will lift up from the dirt and stick to the magnet through the bag. You can then brush the iron filings from the bag into a bowl or container to collect the filings.
To make a simple electromagnet, you need a 6-inch iron nail, 10 feet of 22 gauge insulated wire, wire strippers, a rubber band and a D-size battery. First, strip off some of the insulation from each end of the wire. Next, wrap the wire around the length of the nail, making sure to wrap the wire in one direction. Make sure to leave enough wire free of the nail so you can connect it to the battery. Put the rubber band over the battery so it covers the terminals on either end, then slip one wire between the terminal and the rubber band and then the other wire between the other terminal and rubber band. With the battery connected, the nail should now be magnetized.