Kim Kardashian & North West Make One Adorable Pair of Mermaids In B-Day Bash Pic

The "Keeping Up with the Kardashians" star took to her app on Thursday, where she detailed her daughter North West and her niece Penelope Disick's joint birthday party.

The reality TV star also shared a cute photo from the festivities, where she matched Nori in a mermaid costume (the same one she rocked in 2012 for Halloween, no less!).

"North and P had a joint mermaid party over the weekend," she posted on her site. "They are obsessed with mermaids, so we gave the party a fun 'Under the Sea' theme."

"How cute is this mermaid costume North had on?!," she wrote about the photo above. "I had to bring out my 2012 Halloween costume, when I was a mermaid. I can't believe it still fit!"

It definitely looked like her and Kanye West's daughter was super excited to see her famous mama in the gorgeous getup too. "North was in awe, and I knew I would get major 'cool mom' points for wearing this," she added.

The girls' birthday party was filled with adorable sea creatures, a bubble show and balloon sculptures too.

"Heal the Bay sent over an aquarium with a crab (North swears it was Sebastian!), starfish and so many other amazing sea creatures," she wrote. "We had a huge bubble show by BubbleMania, story time, incredible balloon sculptures by Balloon Celebrations, and real Aqualilliesmermaids swimming with the kids!"

There was also a ton of good food ... which prompted Kim to cheat on her diet for the day.

"Hansen's Cakes provided yummy cakes for the girls, and I even had some! I totally cheated, LOL. We also had ice cream sandwiches by Coolhaus, more snacks by Earlez Grille and adorable cake pops by Jasmine. And Lynn from Diva Body Art Glitter Tattoos gave all the kids temporary glitter tattoos!" she added.