Product review (5 stars): A way to fix these wheel assemblies for good and never have to buy again!!!
So sick of these wheel assemblies falling apart.  Really should give one star but will do what others suggest doing so I never have to buy these wheels again. Went to Lowes and got the parts that others were kind enough to post on here. Will replace the cheap plastic screws with stainless steel.
Here, with parts numbers from Lowes, is all you need:

For each wheel assembly, buy 2 of these (we went with stainless steel):

Hex bolts
1/4-20 x 3/4" long
Lowes #396434

Flat Washer
1/4"
Lowes #311726

Stop Nut
1/4"
Lowes #114172