This was raised on another thread but isn't directly related to the thread so I don't think this is TAAT. Also if you read that thread please don't talk about it, this is a wider question than that situation.
Providing everyone gets on and is happy with it I don't think it is inappropriate for a parent to share a hotel room with their opposite sex child/Young adult. I would go further and say (again providing both sides are happy) that it isn't inappropriate to share with your child at any age.
YABU you shouldn't share with your older child
YANBU, it is a parent child relationship of course it isn't inappropriate.