If going during December, book key excursions not included with a hotel in advance. Otherwise, book excursions at least 24 hours in advance.
Depending on your kids, give yourself plenty of time to play in the snow and sled. See if the hotel includes snowsuits/boots as part of your room charge. (Many excursion providers offer these as well.)
Unless you live in Finland, this is part of a multi-day adventure. However, each part can be adapted to fit your needs.
Lapland has great adventures regardless of season. Santa is there year-round, so it depends on how much snow and sunshine you want to see. Most people go in December. Our family chose to go in February because the sun doesn't really come out in December (and there's almost no darkness in the summer). In February, we had up to 8 hours of daylight and the temperatures weren't too cold. Being there a
Go during the off-season. We got a package with our hotel that included food and some excursions. When priced separately, the package was clearly the way to go. When we booked other excursions and just asked if there were any discounts available, we sometimes got one.