Hi Jayne, how fantastic that you're bringing your daughters on their first trip to Disney World and how exciting that you're celebrating a Birthday too. This is the year of "What Will You Celebrate?" at Disney so you'll find lots of exciting extra things such as the Move it, shake it, celebrate it parade that your girls can join in with. Firstly I recommend you pick the Birthday girl up a Birthday badge (Americans call them buttons) so everyone knows you're celebrating. You can get your other daughter a first visit badge so she won't feel left out. You can get these when you check in at your Disney Resort or at any guest services in the parks.
There are some great character meals in WDW which would be perfect for a Birthday meal, you can see all the choices by choosing Character Dining from the Dining Experience drop down box
HERE. A few of my families favourites are the Crystal Palace with Winnie the Pooh and friends, Akershus or Cinderella's Royal Table with the Princesses or Garden Grill with Chip & Dale and pals. These can be booked up to 90 days in advance by calling 001-407-939-3463 make sure you let them know it's your daughters birthday when you call! You can also order a cake or purchase a party bag whilst you're there, you can see details
HERE.
You can arrange a special present for your daughter through the Disney Florist, but to be honest at her age they may be too elaborate and expensive so how about buying her her first set of Mickey ears with her name embroidered on. You could also visit the Main Street Barber shop in the Magic Kingdom where they will put glitter and confetti in her hair.
Those are just a few ideas, if there's anything else I can help a fellow Brit with please feel free to ask me a question directly via my profile page,
Jo