Check the weather report and trust it - we went last year and were wrapped up to minus temperatures, it was unseasonably warm at 0 degrees and we were very over dressed.
Take waterproof gloves and spare gloves / socks for snow play.
Getting the kit on (boots and suits) is hectic - make sure you have an easy access bag to chuck your stuff in to so it doesn’t get lost (the boots are tight - if in any doubt size up)
We took head torches and finger lights for the children so we could easily see them - the head torches were pretty useless as the batteries went quickly in the cold, but the fingerlights were amazing (we took a lot)
It was amazing - expensive and exhausting but so worth it! Have fun