The one thing you mentioned in your post but, in my opinion, didn't emphasize enough, is the fact that you have a co-founder helping you with this. I've found

The one thing you mentioned in your post but, in my opinion, didn't emphasize enough, is the fact that you have a co-founder helping you with this. I've found that it's a much easier sell a side project to the family when you have a team mate believing in this idea and willing to work at it with you. That early "validation" goes a long way in justifying time away from the family to spend hacking/coding/selling.