We've figured it out: North West is totally that friend who turns her one special day of the year into a week-long festivity. And to think, she's only 3 years old.Kimye's little girl spent her actual birthday, the 15th of June, at Disneyland, where she rocked an Aurora costume . Apparently, that was just some minor pre-gaming before the main event, which was obviously yesterday's major mermaid-themed blowout celebrating both her and cousin Penelope Disick's birthdays.Much of the Kardashian-Jenner crew came out to toast the birthday girls, who wore mermaid tails and teeny shell bikini tops for the soiree. Auntie Khloé Kardashian got into the under-the-sea spirit by donning a lavender wig. Auntie Kylie Jenner, meanwhile, posed with Ariel.And what about the mamas? Kim told her fans she'd be channeling her own mermaid emoji, while Kourtney seemed to be auditioning for Baywatch. Wouldn't it be fun if Kanye went as Ursula?Enjoy the party pics below, and spare a thought for the poor, unfortunate souls who weren't on the guest list.