This spinner uses a standard 608 skateboard bearing in the middle hole, and 14mm steel ball bearings as weights. The ball bearings can be pressed in by hand.
Print two of the caps to place inside the center - if you find they're too tight scale them down a few percent and print again, and if they're too loose just glue them. You can also make custom caps here.
I found the large 3-ball spinner was too big for kids' hands, so I've included a smaller 2-ball version for kids.
I recommend these bearings.
20% infill and 4 top and 4 bottom layers for the spinner.
100% infill for the plugs.