We went Lapland in December 2015, our DS were 4 and 7 at the time which was a perfect age. I wouldn't take a child who's still in nappies as it's a pain with all those layers but from age 4 it's fine.
We stayed in Rovaniemi and went with Thomson. I'm from Finland myself and I was slightly worried that it'll be naff and cringey what with being bussed around with other tourists in my own home country but it really wasn't. It was organised really well and it was just amazing holiday. We stayed for four nights I think.
I was also worried that the Santa would be too "coca-cola" like but he was rather home spun, serious yet kind yet very charismatic. It was a one to one and not rushed at all.
We did husky and reindeer ride but not with Thomson (ridiculous prices). Instead, we went to the Santa Claus Village on our "day off" and they have independent husky park there as well reindeers and many other things. The husky ride is amazing fun, I could do it all day long.
I haven't been to Saariselkä yet but we are travelling there next February half term to try skiing. By the sound of it, my boys will love the toboggan ride!
Oh and one more thing, we usually book our travel independently but going to Lapland wasn't worth it. If you fly to Helsinki and then change flights to any Lapland destination, it will eat a huge chunk of your time. I would have loved the sleeper train from Helsinki but it's eye waveringly expensive. There wasn't any money to be saved.