Well, I can't say which is better for a 6 years old, because in general kids are happy when parents are happy. All the activities you would imagine to do in Lapland would be great for your kids too (except for ruska tourism, because that is different). Just pick something you yourself would like to do.
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Winter Wonderland Hotel is more commonly known as Bear's Lodge - and by locals as Pohtimolampi (the place has a long history, and we still go by the old name). It's a small hotel located some 30km from Rovaniemi town center, and you could (ok, will) say it's almost in middle of wilderness. There is a husky farm somewhat nearby, and a small village called Sinettä a bit further up, but not much else. But of course Rovaniemi, Santa Village + plus others only some 30 minutes away.
Holiday Club Saariselkä is in a resort of Saariselkä, so it's not totally as "isloated" from other tourists as Winter Wonderland Hotel is. And it's a proper good-sized hotel. Saariselkä is a smallish ski resort even by Finnish standards, but on a plus side it is much further up north than Rovaniemi. If you don't have something included in your package, all activities will be there, and especially in December it is a more snow-sure destination.