It sounds like electric baseboard is the most feasible option in this scenario--not quite big enough for a mini-split. As far as sizing goes, I'm not sure. Perhaps someone else can fill you in on sizing. While electric baseboard isn't the cheapest method of heating, I don't think 1 baseboard will be overwhelming.

Thats what i was thinking to. I cannot imagine the heater running more that a few hours aday at most. Onces the baby is older and the room doesn't need to be as warm it will probably never get used.

I suggest a 1000 watt baseboard heater. If possible, place it on the coldest wall.

I have used an oil-filled electric heater that looks like a radiator in several houses over the past 10 years. (I have gone through 3, but only because I have moved a lot and some houses did not need one) It works fantastic, keeps the room a wonderful, consistent temperature, doesn't use much electricity and easily stores away in the summer. While you can turn them on and off during a day, they work very well and efficiently if left on all the time. It takes less energy to keep the oil heated than it does to turn the heater off and on a few times a day. I like my rooms very warm and as long as the room had one regular-sized interior door (the heater has heated the bedroom nicely even with the door 1/2 to 3/4 way open, but the more closed the space, the warmer it will be), I haven't needed to use the heater at more than 1/2 way to the highest setting. I live in Colorado, so our winters can be very cold (to give you a reference point for temperature). It also takes a little while to heat up, but once it is warm, the radiant heat seems to fill the room rather than one small area. This is my favorite type of heater I have ever used.

Oceangirl,
FYI, turning on and off does NOT reduce efficiency of an electric heater and, indeed, contrary to your opinion, would SAVE energy using a thermostat. However, I do agree that the oil-filled units are very nice.

Mzellers,
If you have a 1500 watt heater already, you could run a test to determine your requirement.
The test would be to to run this 1500 watt heater at night continuously until the room comes to some temperature and record the outside temperature. Let's say, for the test, the outside temp was 50 deg and the inside room tempure went to 65 deg after 5 hours, then the temp difference of 65-50 would indicate a requirement of
1500*(70-10)/(65-50)=6000 watts
where the 70 and 10 numbers are my assumed required inside temp and design outside temperature.
The formula is no mystery, since it states that the heat load is proportional to the temp defference between the outside and inside temperatures, which has been known since Newton's time.
This an easy test that can be done at ANY time of the year and at night where solar interference is absent.

If you would like to know exactly what size heater you need and know a few details about the room's dimensions and insulation, I recommend using a Heat Load Calculator. The calculator will tell you exactly how many BTUs/Watts it will take to heat your child's room. If you do not know specifics about the insulation, I recommend underestimating. That way your calculation will allow for a heater that has plenty of power to heat the room and not too little. I have included a link to a real good Heat Load Calculator.

I hope it helps.

Here is a link that might be useful: Heat Load Calculator

General rule of thumb is you can expect 1000 watt electric heater to warm up a well insulated 100 sq ft. room ( 10 by 10) Just multiple the heaters wattage by 10% to determine the amt. of sq. ft. it can heat. We had a similar situation as you and decided to use a portable heat pump. You will either need a window or make a vent through the wall for the exhaust hose. You can expect these units to use approximately 1/2 to a 1/3 of the power of a electric space heater. Most, if not all heat pumps, have a electric resistance back up for those really cold days. Both electric space heaters or heat pumps will cost more than almost any other heating option (gas, oil, wood, etc.) However, there really cheap installation can make these your best bet.