I am all for leaving the bad and jumping into new territory with a positive outlook. It worked great for me. Left a stable job, moved across the country and found a job I love. I am much happier.
Things to think about with the job opp: you can get by without 401k and tuition for now. Medical is a big issue though. With two kids, your medical insurance costs can get really high. You need to see how much it will cost to keep your family safe, healthy, and not get stuck in a financial sinkhole from a big medical bill. Also, being a startup with only 4 other people, you may be working long hours. Are you willing to do this?