I've known from an early age that I wanted to work for myself, and I've structured my life to make that possible. I've always had side projects going - through high school, college, and two full-time jobs.
When the side projects grew profitable enough and we had socked away enough money, I quit my job to work on my business full-time. My wife was about twelve weeks pregnant when I gave my notice.
That was a bit over a year ago, and I haven't looked back. I'm financially conservative, so I probably waited longer than others in my shoes would have. I had enough income to support a basic lifestyle (I call it "pork-chop profitability"), and if the income dried up we had enough savings for a couple years. It was a high bar, but with a baby on the way I didn't want to be reckless.
Start working on small, attainable side projects with immediate revenue potential. Pick two targets: self-employed income and non-retirement savings. When you hit both targets, it's time to go full-time. And most importantly, make sure your family supports you and understands your ambition.
Best of luck.
I watched a rather regular guy launch a non-tech company in the middle of a divorce with primary custody of two very young kids. That company hit 5 million a year in just a few years after launching. Seriously. He just thought his current employer sucked and that he could do better, and he did.
Have I done anything like that? No. But my iPhone apps are making me enough money that it's like having an extra part time job. I started in August, with only a maximum of about 2 hours a night to work on them.
You need to think small. Really small. You need to actually finish those projects.
You could consider switching to contracting. If you are good, you can squeeze the same income into a shorter work week. Haven't done it myself, though.
I now have a side project that is growing pretty consistently. It took me at least a year to realize that this could really be something. Over that time there have been periods where I have given up and not put much time into it. So far I've always come back. I can now look back and see that the months I gave up revenue stagnated. I can also clearly see what I did that increases revenue. I can also see that if I keep working at the same rate (some evenings and some weekends) and revenue increases at the same rate that it should grow into something that will replace the 9 to 5 in the near future. That is particularly exciting to me as I want a business without the stress, complications and long hours. I now look forward to not working full time on it.
So keep at it. Something your working on now maybe the solution. It doesn't have to be big and exciting. There's plenty of people trying the same. It's not easy for sure but certainly doable.
Projects 'on the side' is renting yourself out hour by hour, it does not scale very well but if a side project is sold as licensed software (niche software more or less by definition) you could make out very well and if someone wants to license your source code instead of the final product you could make a big chunk in one go.
It's quite difficult to say what you can achieve without having a good grip on your situation and your skills but if I had to start over again from a day job I would do something like that.
You should consider creating something that individuals or small businesses will pay for because it scratches an itch. I pay for several services like that each month -- Blinksale, Springloops, AuthorityLabs, Chargify, WebsitePulse, Postmark, and Basecamp. (But don't limit yourself to tools for developers -- there are many industries with un-scratched itches.)
Essentially the short version of 4-hour workweek.
If you leave and the pain of going it alone is more than you experienced at your 9-5, and you aren't feeling fulfilled, then you have gained a very valuable bit of insight. Same deal if you have the time of your life.
Only way to know is to just do it.
But as you have kids-- its not about you as much anymore.
Get smarter about your job, become indispensable, hold your nose and kiss up without shame, build your cushion, then jet.
You could also spend the time you have working on the idea and crafting a (tested) vision, then outsource the actual creation of the product with funds from your 9-5.
Then jump ship.