A trip snowboarding is an adventure I would love to share with my ABG
I would put her to bed the night before in her footed pjs and night nappy, then carry her to the car wrapped in blanket really early for the drive to the mountains, so she can just sleep in the car.
Under a ski suit no one would ever know she was in a nappy or trainers as everyones pants are bulky and noisy at the snow.
Plastic pants would be a must however as mentioned below I am not sure the outer plastic of a disposable would handle a butt slide down some hard snow / ice even under ski pants/suit.
The extra padding would come in handy too for those stacks she would also be the only beginner snowboarding with a warm dry butt on the mountain as no matter how good your ski pants if you spend lot of time on ya butt your pants get wet and stay wet.
With her legs in the board bindings I don't think a nappy would be too un-comfy as the bindings keep your legs apart.
A cute pink 1 piece ski suit would be a must privatina makes them, also a helmet with what looks like little teddy bear ears poking out the top.
I am not sure if it is the case in other countries but the mountains near me in Australia the lift pass is a sticker that is stuck on a bit of wire. This wire allows the lift pass to be connected the pass to a zip or part of your ski gear that is visible, cutting the wire voids the ticket.
It would be kinda cute to put the wire through both zips in her 1 piece ski suit, in doing do sealing the ski suit shut, she would know if she wanted her ski suit off it was the end of snow play for the day.
I think booking her into the ski school would be kinda cool too.
Adventures such as this would be great fun, with every effort taken ensuring that the fact she was in her nappy was our secret and remained that way ensuring the chance of someone finding out was remote as possible.