I've got an idea that might work for 13 year old girls.  You can have a princess theme party. Don't ask the children to wear costumes ahead of time.  The idea here is that you'll turn them into princesses by the end of the birthday party.  That way it becomes a costume party, but the parents don't all have to go out and buy princess Halloween costumes ahead of time.  Here are some of the activities you can to:

1. Apply princess makeup - get some sparkly eye shadow and powder/blush.  If you apply lipstick, apply it with cotton swabs.

2. Princess up-do hair styles - all you need is a bunch of bobby pins and some glitter hair spray.

3. Have a crowning ceremony, where each girl gets a princess crown (doubles as a "treat bag" item).  This is a nice princess crown:

Jamine Tiara - Disney Princess Collection

4. Have a scavenger hunt type of game, where the girls have to find these items that a princess would want:  a wand, a prince (a picture will do), a frog (to have as a pet), a fairy (could be a small statue or a picture), a mirror (could be decorated in a pretty way), whatever else you can think of...

Decorate with the princess theme - here are some items:

Disney Princess 18" Foil Balloon

Disney Princess Pull String Pinata

You may want to run these ideas by the parents first in case there is any objection to the makeup, hairspray, etc.