I'm a Northeast Corridor native living in Asia now. I've lived outside of Boston, in Manhattan, and in Georgetown. They are all great places with their own merits. Boston is GREAT for children. I have twin 2 1/2 year olds that I bring back to visit my parents. Except for the cold weather, I'd move back there in a heartbeat. There is a fabulous Children's Museum, Science Museum, Swan Boat rides in Boston Common, Drumlin Farm, all sorts of public playgrounds in the Greater Boston area, YMCAs, you definitely can't go wrong. Boston also has amazing medical facilities everywhere. I'm assuming your husband would get medical insurance coverage through his company. Should you need fertility treatment, Massachusetts requires that health insurance companies cover treatment, including IVF.
My parents live in a wealthy suburb called Newton, just outside the gates to Boston College. There is a T (as the public transportation trolley trains are called) station a five minute walk from where they live. It brings you straight into Boston. So you can live in the suburbs and still have easy access to the city. Lots of New England excursions (mountains for hiking and skiing, beaches, foliage, quaint villages) to be had.
Washington, DC is lovely too with all the free museums & monuments. I was there in university so I can't say how good of a kid town it is. The public transportation system is good. Georgetown is lovely but there's no underground there. There are lots of nice excursions into Virginia, Maryland and West Virginia. The weather is quite mild, I found it perfect.
NYC, well, it's just an amazing place. I suppose it's a bit like London in that there are all kinds of people from everywhere, great restaurants, world class entertainment and cultural venues. Rent is quite high, but not as high as comparable neighborhoods in London. Brooklyn Heights, Park Slope, Cobble Hill, and other Brooklyn neighborhoods are all options, but I think if you're only going to be in NYC for a short while (like two years), live in Manhattan and visit Brooklyn. Excellent healthcare available here as well, though I don't think health insurance is required to cover fertility treatments.
Having lived in all these places, I would now choose to live in Boston because it's a great place to bring up children.
I think Montreal is a lovely city. I have several friends from Toronto who all love it (okay, they are all expats so they can't love it that much). I personally would go absolutely crazy with how long the winters are there and how damn cold it gets! I'm used to Havana-like weather now and just don't want to go back to the deep freeze.
My two cents.
Good luck.