I travel to both cities frequently. Especially in light of a five-month-old, perhaps you would consider just staying in Buenos Aires. It will Be a more relaxed vacation. Remember there’s nap times changing’s etc. and your wife would probably be looking forward to some down time. While Montevideo is an interesting city, it’s not that easy to grasp at a first glance. Until you can figure out the different neighborhoods and what they offer I don’t see it being as conducive to a family vacation as Buenos Aires would be.
There’s so much to do in Buenos Aires your time will go by very quickly. Just a thought…