How do I keep the house warm this winter?

You are very very lucky to have such a wonderful space! Walk around the house and inspect all the doors and windows, those are the two areas where there is the most heat loss. Check all outlets and insulate if necessary. Use thermal drapes downstairs. And invest is a quality space heater for the downstairs to use in the room the baby will be in during the day. Here are some other methods to try.

https://www.bobvila.com/articles/too-hot-upstairs-too-cold-downstairs-here%E2%80%99s-what-to-do/

https://www.hunker.com/13415603/how-to-balance-upstairs-downstairs-temperatures

https://www.artofmanliness.com/articles/how-to-keep-your-house-warm-in-the-winter/

Lots of good sites from Naomie. You don't say what kind of heating system you have. Warm air rises and drops as it cools. On a forced air system...partially closing off upstairs supply vents will force more warm air downstairs. Make sure doors, windows are weatherproofed. Use ceiling fans to circulate the air.

I installed a shower rod in the stairwell and hung a pretty quilt that stays closed during the day. I also use space heaters in the rooms that I am using to bump the heat a bit. The infrared box heaters are safe, cool to the touch and efficient.

Try keeping the upstairs bedroom and bathroom doors and heating vents closed.

Check out this list of tips... https://coloradoclassicexteriors.com/keep-warm

I'd start with the biggest thee sources of issues: check your weatherstripping, fill in window gaps with caulking, or consider upgrading to more weather-resistant windows.