I have a small upstairs nursery with no heating duct (and small ducts in the other two rooms). For a simple and quick fix I thought about cutting a 1' x 1' hole in the wall to the next bedrrom and putting up a nice vent. Will this work? and where should the vent go (high or low)?

Not really. The nature of hot air is to rise pushing the cold air to the ground and
back into the returns. If you cut the hole near the floor then cold air will gravitate to the next room. If you cut the hole near the ceiling then the top half of the nursery will receive some warm air but it will stratify and where the baby sleeps it will still remain cold. For a small nursery I would suggest some supplemental heating source. One of the small inexpensive ceramic heaters would heat a small room in no time and with a thermostat it should not be that expensive to operate. GOOD LUCK!!!!