Hi Emma! Welcome to planDisney.
Magic Kingdom Park at Walt Disney World is the Most Magical Place on Earth to spend a birthday!
Seven days is the lowest available to purchase from the United Kingdom website. Hakuna Matata, it means no worries, as alternative options are available. If pre-booking is a must, you can buy 2-day tickets on the
European site (in euros). Please know the standard tickets available do not include Park Hopper (available as an optional upgrade).
The second option is to buy single-day or shorter-dated tickets for Walt Disney World in the United States. Prices are subject to change and will vary depending on the Park you visit, the date, and various optional add-ons such as a Park Hopper Option. If you choose this option, I recommend buying tickets in the States online or by calling Guest Services rather than at the Main Entrance Ticket Booths, so there is no availability disappointment at the gate.
As your visit is in September, you could book
Mickey's Not-So-Scary Halloween Party. This special event (separately ticketed) runs on select dates from August to October, offering experiences such as Mickey's "Boo-to-You" Halloween Parade and spellbinding overlays on some rides at the Magic Kingdom Park. You can usually enter the Park as early as 4 pm on the day of your ticket, and the event runs late into the night. The dates are not released for 2024 yet, so please check the Walt Disney World site for updates.
I hope this information helps, Emma, and whichever option you choose, you will enjoy an enchanting time at the Most Magical Place on Earth. Don't forget to pick up a birthday celebration button from the front desk at your Disney Resort hotel or various retail locations in Magic Kingdom Park. Cast members are talented at spotting and commemorating special celebrations, so you never know what extra pixie-dusted magic is in store!
Have a magical day, Louise.