Best advice we were given - don't overpack streetwear. You'll likely only wear it for a few hours after you're done riding (usually after a shower) so you can get away with wearing the same stuff for quite a few days.
Personal recommendations for on-slope:
thermal base layer (we have merano wool)
fleece top, especially if you wear a shell
your normal protective kit, especially a helmet
facemask, buff, or scarf
Bring several pairs of snowboard/ski socks, you probably don't want to wear the same pair more than a day or two. Even if you can clean them, you want a dry pair for the next day. Avoid cotton clothing on the slopes, it gets wet and stays wet. Sunscreen can be useful if you're out for more than a couple of days - my face was quite rosy after a week. If you take a backpack, keep it light. I took a low-profile backpack this past season with two small water bottles and a couple of baguettes for lunch. You can try keeping stuff in your pockets, but we found it too bulky.
Stoked for you guys, the first holiday away is so epic!