Technically, there are some metals allowed inside the microwave. In fact some of the pre-packed food you nuke contain metal in it in the form of foil. What matters though is the shape of the metal or object you put in the microwave.
To make it clearer, watch this helpful video I found:
Well, I hope that explains everything!
To be safe… just tell everyone in your home especially the kids to never put any metal in the microwave!