Is there even an opportunity to adequately balance the demands of the new startup vs what your personal life and family are vying for?
I generally put personal and family time on my calendar and treat it like any other meeting on my calendar. It may seem unusual to have "bike riding" on my work calendar, but it works for me ...
There is always the opportunity to balance personal/work life. You can balance it however you want. The result that comes to fruition will depend on how you as a person handle things.
If you spend 90% of the time with family and 10% working on the startup it will take you 9 times longer to get something done than if you worked on your startup 90% of the time and spent 10% with family.
If you are the type of person who can grind day after day, with no support or encouragement from friends and family, eating at your desk from sunrise to sunset, and sleeping on the futon in the corner of the office every night...you could take even the worst idea to market. But most people can't do that and need to "unplug" every once in a while.
The less you need to "unplug" the faster the startup will get things done. The more you need to take a break, the slower things will go and increase the chances of you getting left behind or overtaken by a competitor.
It's your choice.
Consider spending days solely with the family or solely working. I find a 4 working days and 3 family days being a good split. (2 work days, 1 family, 2 work, 2 family)
On days I work, I get up early, spend all day behind the computer, including diner. Go home, sleep, repeat.
On family days, I do not work at all. Instead it's all about quality time. Going to the zoo, the pool, etc.
No. If your startup does not consume you then you have the wrong priorities. How you manage the personal life around it is delicate and important, but there is no balance.
Initially it started out as a trial by fire period, but after understanding what I can and cannot do, I have tried to schedule personal time with family in during the week as I've found it recharges me. All other times are fully dedicated toward the startup.