Me n my DH went before we had our DS , it was the best trip ive ever been on! You will love it, i promise and you kind of forget about the cold, (i am the colest person i know, and even i coped). Go on as many excursions as you can, even the ones you think you'll hate, do it coz they're all brill (and im not sporty!)
As for what to wear, layers as oppsoed to one thick jumper, i did: Bra, Strappy top, t.shirt, shirt, jumper, coat, hat, gloves, scarf etc and loads of thin socks, again layer it up. Dont worry, everyone looks huge.
God im sooooo jealous, would love to take DS when hes a bit older. You will have a great time. Tell us how you get on.
Oh and before you go, look at a website : www.levi.fi coz thats possibly where you'll be staying.
Oh and one final thing, watch out for the bears. Grizzlies. Seriously.
Have fun xxxx