My son's bike had long axles extending out from the sides of the back wheel- great to attach training wheels- but if you don't need training wheels they like to draw blood.

So, I designed these simple axle caps.

I printed these at 15% fill in ABS, flat side down. Depending upon how rough your kids are on bikes you may need to print at a higher density.

Print, press on to axles, line up nut trap with axle nuts, and tap on with a rubber mallet.