Hello chocolate
I was based in Sala which they sadly don't run any more (bloody recession!) but in addition to chumming up to other operators, knew the Canterbury Travel team and resorts very well!
Most of my resort clients were on 3 or 5 day tours with the christmas crowd on a 7 day stint (Christmas day in Lapland is the strangest thing as an adult!) however we did cater for 6 parties of single day trips. It's a different level of experience for each 1, 3, 5 & 7dat trip. They're all totally magical, involve exploring and having a lot of fun- having worked all of them, id personally opt for the 3day trip. You don't get too overtired on a 3 day trip. Some little kids are wiped by the one day trip-early up, airport, hyper excited, snow, playing, more snow, more stuff, no nap, crash!!! Tears. 
3 day trip is more measured... Day1, get in, get excited, eat, kids entertainment in the eve, sleep. Day 2, all go out together and look for Santa, finding his elves and other cool stuff en route, (more evening entertainment!) day 3, try again, find elves AND reindeer and then Santa who knows all about your DC and has a present for them too (which you deliver to elves when "sorting out some paperwork" on day one!) Each of the resorts is (so far as I remember) designed so that it's almost a self contained log cabin filled, stone building, snowy oasis in the beautiful wilderness! You rarely hear anything other than Canterbury Travel as you're so far off the map (it feels!) I can't recommend a resort over any other based on where I lived as my place is no longer avail.... My baby bro worked at Luosto and that's quite large,possibly less preferable Soumu is lovely!!
Don't worry about DP an DC not liking cold. You're all given very very good (boiled clean between clients) all in 1 snow suits to wear when you're out there. All in 1 means no gaps to freeze and let snow in through! There's a hood included but chuck in a bobble hat and maybe a scarf and you're all good. Because of the insanely low humidity, it seriously doesn't feel cold. We regularly went on 6k hikes to the nearest shops that sold gin and the worry wasn't ever feeling cold (we hit minus 56 one week!) it was that being out that long my camera froze!!! (keep all electricals under your suit and your body warmth will stop them freezing!)
The Santas are professional Santas (I snorted when I learnt this but they are brilliant!) they spend a year growing their big bushy beards then shave them off after Christmas and start again! The Canterbury Travel Santas are separate to any other Santas in Lapland. They are in their own log cabins unvisited by any other company, they are in the middle of nowhere (hence hunting for Santa!) and they introduced me to G&T
Like I say, I worked for Canterbury Travel. They are massively strict with their staff (MASSIVELY) you will never see anything you're not supposed to, it's all about the kids and I was impressed by them. Their staff took real pride in their work and treated their staff very well. This was 5 years ago. I appreciate things may change but I sincerely hope they've not. I still hear good things about them!
You're NEVER guaranteed the Northern lights. You will see them if you're lucky. They are stunning but you can go for weeks and see nothing. 
Lapland is the Anti Narnia... It's always Christmas and never just winter!!! 
Hope this helps!!! Did I miss anything stupid memory iPhone doesn't let me look at previous posts when typing!