I would take buggy over a back carrier just because it can get quite slippery plus a buggy means you can go out in the evening and a baby will be all nice and cosy in a buggy and sleep.
One set is fine if your child is not prone to accidents otherwise I would get a spare pair of trousers Lidl/Aldi do cheap ones. Always two pairs of gloves and I found sunglasses with a head strap thingy easier than googles on little ones but am trying googles this time on DS.
Decathlon have some great deals on at the moment for adult thermals but I get my DS thermals from H and M as they are cheap and keep him warm and aren't white!
Are you driving? If so then pack up basics herbs, spices, tea, coffee, hot choc, pasta, rice, tom puree, part baked rolls that kinda thing as it is very expensive in resorts and milk. We took UHT as cheaper here than over there.
Ear hat thing that just goes over your ears...no idea what it is actually called. Very good because sometimes you can get too hot for a full hat but your ears get cold on the lifts.
A little back pack to chuck your layers in and some water. Pain killers because if it is your first time falling can happen a lot!
Glove liners and proper ski socks. Comfy pants because having horrible uncomfy pants on with salopets is really really annoying.
Sledges far cheaper in decathlon than in a ski resort but obviously easier to take if driving.
Have fun it is great and I love it because I am throwing DS in the ski school and creche all day this time and will be skiing all day like a care free person......well until 5pm anyway
Still it is the only proper break I get in the year.