How to collect wages owed from a painting company, llc?

My son and stepson had been working for a painting company in Kansas City, Mo. My Stepson worked for them for about a month, my son just for one day. Two weeks ago, they were notified that there was no more work and would be notified when work would be available. The owner stated that he had sent the wages owed, about $ 600.00, to my stepson and $ 100.00 to my son in the mail. Odd thing is, the owner said he put all of the wages on my stepsons check. That didn''t seem right. After two weeks, nothing has showed up. They have both attempted to call the owner, and he will not answer his phone or return messages. Both are over 21, if that matters. I''m just trying to help them out. The company is _______Painting, LLC. We are not sure what the next steps should be, or if it is even worth the time pursuing. It just get''s under my skin that they provided the work but might not get paid for it. Thanks in advance for any help. John