By the time you get into juniors, you’re probably a size 2 or 3. If you wear a size 12, you’re a size 0. If you wear a size 14, may be a 1 or 2. Some stores have a 00, if they do you’re probably a size 10.
The only problem might be the length. It will be a little longer when you go with the junior size 0 than it would be with a size 12. With capris and shorts it will not make a difference.