"Axle cap": what is it called and how to remove it?
I assembled toys, displays, and bicycles for a major retailer for 7 years.  I despise removing those caps in a recessed hole like the center of a plastic wheel.  You can snap the plastic cover off, then use vice grips, pliers, a screw driver, or wire cutters to twist, bend, pry, or cut the metal part to remove it.  They're almost always destroyed during removal so expect to use that spare to replace it.