I've done some LF shooting, mostly studio, and have tried to combine family outings and photography with 35mm and medium format. We have a 2 year old, so the advice above to have him be more patient is rather amusing. My solution to your dilemma would be very simple - forget about LF entirely when you are with the family. What's great about large format is that you have movements and can get much better control of the image, and you end up with a large piece of film.
However, 6x7 on modern emulsions yields extremely high quality enlargements, so I think you will rarely benefit from the added film area unless you are tripod mounted and quite deliberate in your focusing and shooting style. That's completely unrealistic for me in a family outing. Movements are great, but they also take a lot of time to fiddle with in combination with precise focus; again, an exercise in frustration and annoyance all around. So I see very little advantage. A 6x7 rangefinder like the Mamiya, on the other hand, is a totally different situation. While my best photographs are almost always a result of taking my time, if you need to you can take shots very quickly with the Mamiya. And if you are using 220 film, you don't have to reload too much more often than with 35mm. The camera is portable, fairly light, quite rugged, and has superb optics. So I think it's the best compromise for you, given your objectives.
My own approach is different; I choose to shoot 35mm in such situations, not least because when I am with family I want to take pictures of them. 35mm auto-focus has yielded many more wonderful images of our 2 year old than my Hassy, simply due to the fast handling and focusing. I'm reasonably fast with medium format and use it a lot in the studio, but modern AF gear is far quicker than I am. So I use MF (or sometimes, but rarely, LF) for my deliberate images and for studio use, and 35mm for pressured situations.
Cheers.
Bookmarks