Unemployment solved: 20-hour work week

While watching yet another presidential candidate promise how he would stimulate the economy to create more jobs, I found myself wondering why nobody is talking about an obvious solution: restricting the supply of labor by cutting the working hours.
 
Given the current productivity levels and 5 billion people of working age on the planet, it does not seem possible to give everyone a 40-hours/week job doing something useful.  There is a precedent for reducing working hours.  We went from 70 or more to 40 a while ago.  We also eliminated child labor.  Was it a coincidence that real wages went up and the middle class got created since then? 

Why is 20-hour work week a bad idea, or a good one?

Cutting the work week to 32 hours doesn't change the fact that it would also reduce people's pay by 20%.  Once again, there are scores of people in this country who either can't make ends meet now or are just barely getting by.  Cutting what they make per week by cutting their hours WILL NOT HELP THEM.  It's not a viable solution and will only make things worse.