You have to be a Princess in your own right, i.e. Princess Anne, Princess Margaret, el al to use your christian name. A commoner can only become Princess 'husbands name'! i.e., Princess Michael. Meghan can call herself Princess Henry, Kate, Princess William, Sophie, Princess Edward and so on. I guess this is why they are given dukedoms on marriage, then they can become the Duchess of ? Children can become Princes/Princess's only if their father is a King/Prince, (if they choose to) but not if their father is a commoner, even though their mother is a Princess in her own right. Again, Princess Anne's children and Princess Margarets.