Its In The Bag has these clever kid’s bicycle gift bags that hold any children’s bike with up to 20″ wheels.  They’re recyclable and reusable, so you can save them for future Christmas’ or holidays for other kids.
Just roll the bike into one of three bag designs and seal it up with the attached velcro strips.  £6.95, no word on US availability, but I’m sure if you inquire, they could ship you one.  Now you don’t have to try to gift wrap the bike…you just have to put it together.  They have a birthday design coming soon, too.