Typical air leakage areas are outlets and light boxes (wiring holes in them), around the windows (behind the trim) and where the walls meet the floor (behind the baseboard and carpet edge). With the exception of the wall/floor joint, most of the leaks can be fixed with latex caulking. I use white or off-white with light colored trim, or if I will paint anyway, and clear for many other areas where color matching will be difficult.
You can install gaskets behind switch plates and outlet plates, but this is not as effective as sealing the wire penetrations in the wire boxes themselves. If you are comfortable with wiring, caulking the wire holes is a better solution. If you're not, just use the gaskets.
If the bath fan is on a timer, you may be able to adjust the time it runs. I might help to keep the doors to these spaces open during the day, and then close them at night. A space heater is probably a good solution, I would recommend an oil-filled electric radiator over a heat/fan style for noise considerations for your 6 month old, unless the fan would be helpful for white noise. Make sure the bedroom's and the home's smoke detectors are in good working order.
Since the rooms are over the garage, minimize the heat loss in the garage as well via similar air sealing and gasketing the vehicle door in the garage.
