I'm just a dad with 2 kids(2yrs + 4yrs)
I agree with you on spending another $3200 2nd 5D III that might not be use as much as my first. If I do buy 2nd body, I want to make sure the 2nd body will serve the purpose.
I don't think you should worry about not using the 2nd body. I thought the same when I got the 6D but since the day I got the 2nd body, I've been using both bodies together for whatever I shoot. I can't imagine myself going back to a single body system now.
The convenience of shooting with two cameras without going through the trouble to change lenses far outweigh the trouble and cost of a 2nd body - you'll use the 2nd body extensively, only you don't know it yet