Indeed, one must keep the box and the contents — they are both artifacts. If they are not to be stored together, it will be easier to give each a consecutive number. Relate the two objects in the description field of the cat card.
Plastic dolls and other materials are the latest conservation and curatorial problems. Plastic is a petroleum byproduct or a chemical chain. Plastics in collections are starting to break down. (Barbi and the boyfiend are plastic — on so many levels). National Park Service has written advice re: plastics. Instead of giving the complex link, just google Conserv O Gram and you will be taken to the site. Plastic conserv O gram is number 8/4. While there look at the list of conver O grams. It is extensive and excellent. You will find great assistance at that site and can print off the documents for the price of paying your taxes.