We went to Saariselka, travelled with Esprit (think the Lapland side is called Santas Lapland). Our hotel was part of the Tunturi, the Gielas wing. It was just fantastic! So well organised, not at all commercialised just very very Christmassy and truly magical.
The 'Santa' day was just wonderful.
Ds was 5. He still believes now (he's 7) and I think going helped keep the 'belief' (and magic of Christmas in general).
It was very tiring as the flight was early, ds was a bit of a pain as he was tired. If you can, I'd stay an extra day (we did 2 nights/3 days) as an extra day would give you chance to catch your breath. There is a fab toboggan run, which we did the day we got there but didn't stay long due to being knackered from journey! Would be nice to have more time to walk round the village.
I read some bad reviews about Canterbury travel who were recommended to us, I think the recession hit them quite badly and the service suffered.
You need to avoid Rovaniemi as that's the commercial one. Get as far North as possible.
I wish we could afford to do it again, it was fantastic. So snowy and pretty, but wild too, you really feel like you've been to the North Pole!