Semi-retirement for people in their 30's and 40's

Lately, as I've been rereading "Semi-retirement: Live More, Work Less" by Bob Clyatt, it occurred to me that my wife and I could reasonably get to semi retirement by age 42. Currently we save between 10 - 15% (plus employer match) of my income. We are going to have our third child here in November, so theoretically if I continue to save about 15% of my income for the next ten years, my wife returns to work in five years, and we save 100% of her income for retirement we could have about a half a million saved in our retirement accounts in 10 years. Then by age 42 both my wife and I could easily work part time (20 hours per week), and save 15% of our income for full retirement. For me this would be ideal, because as our kids grow up it would leave me more time to spend with them and possible be their coaches in some of their extra curricular activities. Plus have 20 more hours a week of free time would allow my wife and I to freely explore other alternatives of income, which could lead itself to possibly leaving our regular part time job for something we enjoy more. Basically, a semi-retirement scenario would free us up to see more of each other, our kids, and our family and friends. Now its just a matter of setting this into a tangible goal that will be achievable and hopefully one day realized.