I've put enough kids toys together to know that you need an axle top hat nut. Google that and look at the images. You can also buy retainers which have little find that grip the axle. Most new toys have a plastic end cap with the finned retainer inside it.
Top hat nut:
Axle retainer:
BTW - to install these you need to stick a socket under the fixed one, then you use a deadblow hammer or rubber mallet to tap the other end on.
Top hat nut:
Axle retainer:
BTW - to install these you need to stick a socket under the fixed one, then you use a deadblow hammer or rubber mallet to tap the other end on.