To be honest, I can relate to your situation. I also have gone through the acquisition phase of both children and cameras. My solution was to focus on my family (first responsibility!), while exploiting every opportunity I could get to earn some income from my photography. On average I pocket about $1500 to $2000 per year from photographic work. It is not much, but it means I can buy my gear without having to "steal" from the family finances. I also do a lot of bargain hunting, and only buy when I am sure of an absolute bargain, i.e. I could sell it for the same price or better if I had to in a pinch. Children are small only once for a very short time, but used cameras will always be there. You may later regret not focusing your devotion on your child when it is still small. I never regret a missed "bargain", because there are always more of them.

How sure are you that you need a Hasselblad per se? I mean, there are other options too, like a 645 system or perhaps Pentax or Mamiya 6x7 systems as well. They are generally more affordable. I got a Mamiya RZ Pro II with 110/2.8 lens, two film backs, extension tube and a few other minor accessories, for $500, and I have taken beautiful photographs of my children with it. That has pleased my wife to the point where she now lives quite happily under one roof with the Mamiya. In her office hangs a darkroom print of each of our children, and in our house there are plenty more. The point is that if you get the MF camera, you also have to produce something with it, that either you get paid for, or that contributes to decorating your home. I don't think your wife could have much of an issue with it then.