Pickwick, Oliver Twist, Nickleby and other Dickens Characters are the  faces in the mirror Dickens sees when he is in his office trying to write. These characters have little dialogue, so you can use these roles to cast kids who are more experienced singers than actors. Since the  other Dickens characters only sing choral parts, you can cast as large a number of kids as you like/need. For fun, you could assign all of the kids in the chorus names from a variety of books by Dickens  or let them research and pick out their own. If you have a smaller cast, all of these actors could be double cast as Carolers, High Society, Street Vendors, and Restaurant Workers.