Considering going part time but scared about the money

We crunched the numbers and could technically live on my wife's salary alone if I went to 3 days a week in the office. I'd pick up the kids from school, help with homework, be less stressed. But what if I fall behind at work? What if I can't get promoted? What if the economy tanks and we need that extra income? Has anyone actually made this leap or am I just fantasizing about something that won't work?