Depends very much on the kids - we went two years ago when they were 7 and 4.
My advice would be to avoid doing a one day budget special - we live 30 minutes from the airport but we were up at 4am and back home at 1am, and we were knackered.
At this time of year it is very dark up there - we landed about noon local time and the sun was just setting, so by the time we got to the bit where Santa was it was pitch black, and remained like that forthe rest of the day. The dark certainly spooked the little one.
It can also get very cold - they give you the kit, but it was -28 when we were there at the end of December, and even the locals were saying it was colder than normal. When we arrived back in the UK it was a positively balmy -6!
I would also point out that the weather can be grim as well - absolutely no problems up in Lapland, but we flew the day Heathrow and the rest of teh UKs airports were shut when the planes froze to the tarmac - luckily for us East Midlands was the only airport open in the country but they were struggling to fit us all in, which delayed our departure and landing coming back
It's a great trip, but if we were doing it again we'd spread it over two days, and go earlier in the winter so there was more daylight