Rent two hotel rooms, one for the girls, and one for the moms. Decorate the girls hotel room with streamers, balloons and a happy birthday banner. You can either keep the destination a secret from the girls (but not from the moms!) and pick them up, or have their parents bring them to the hotel. Depending on the hotel you pick, and what it has to offer, the girls can swim, play mini golf, go to the game room, do karaoke, or any other activities the hotel offers.
Dinner can be as simple or as elaborate as you want and can afford. You can treat everyone to a meal in the hotels dining room, order room service, bring in pizza, fast food or some other takeout, or just bring platters of cold foods from home. After dinner the girls can watch TV, play games, give each other make overs, or any other activity typical of a sleep over. Be sure to tell the hotel you'll be needing rooms that aren't too close to anyone who actually needs to sleep. Take everyone to a big breakfast the next morning, and then send your guests on their way.