You need to tell them the truth, but you should consider waiting to tell them until they get home so they can enjoy the rest of their time in the mountains. Think about how this is going to affect them: the only thing worse than finding out your car was wrecked is finding out when you're on vacation and having to come straight home.
But either way, you need to be honest. You will get punished big time, but you deserve it - don't think you can take a huge risk like that without having to pay the consequences when you lose.