Will you have washing facilities?
We always stay in self-catered accommodation, and I can do washing daily.
So we take 2 sets of thermals, socks and fleeces for everyone. We take one pair of ski trousers and one jacket for everyone and one pair of ski gloves. Maybe 3 pairs of ski socks each, actually. If no washing facilities, I would take at least three lots of underthings each.
I would suggest you take two pairs of ski trousers for your little one, and you may wish to use pull ups during the day, even if they are usually dry, as it can be a bit of a palaver finding a loo, and then undressing. I would also take extra ski gloves for the little one, as they will tend to get quite wet.
Some people hire boots, skis and helmets. We take our own. (We have calculated that by passing down, it will be just as cost effective as hire.)
You don't really need snow boots, unless you use them here. Wellies are just fine. Take ordinary hats and gloves for walking around the resort in the evening.
You may also want to take some of those flat sled things with a handle. They don't take much room, and are lots of fun.
Have a fantastic time!