Hi OP, seeing the Aurora was on my bucket list so we did lots of research - and obviously got lucky too! We saw them 4 out of 5 nights of our trip, incredible bright displays of yellow, green and pink dancing waterfalls of light to which the photos do no justice.
I honestly wouldn't go at Christmas - the Aurora activity relates to solar flares, and these are measurably higher during winter and spring equinoxes. We went for the spring equinox, around 21st March.
We flew to Helsinki, spent a night there for city fun, then got the tiny plane up to Ivalo and car transfer to Kakslauttanen. We stayed in a log cabin for 3 nights and glass igloo for 2 (preferred the cabin, honestly, though obviously seeing the lights through your glass ceiling is fun!)
It was extremely cold, -30 I think, but very clear blue skies. During the days the activities were fun - we did a dog sled ride, visited a Saami home, reindeer safari, etc. The sauna in the log cabin was great and we used it multiple times a day to warm back up! The 'resort' is not at all resort-y and was lovely.
There is very little light pollution and the displays were spectacular. I'm hoping to go again once our kids are big enough to handle the weather...
Hope this might help - wish you the best of luck.