Hi. I have a MODY which is a 50% heritable dominant gene version of diabetes, if you have a strong family history of diabetes you can be tested for it, but although I am pretty sure I have MODY (mother and brother have it, onset usually when young) they have not identified all the genes so I haven't had it confirmed through genetic testing.
I am pregnant with my second child, although each child has a 50% chance of getting the big D (so chances are at least one will have it) I never thought of not having children, as diabetes is pretty manageable now and it's not the worst thing in life.
I wish you and your wife luck with your decision though......you say it is 'her' decision but surely it is something both of you need to decide?