Kardashians Celebrate North West & Penelope Disick's Birthdays With Mermaid-Themed Joint Party

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Happy birthday, North West and Penelope Disick!
The Kardashian-Jenner family celebrated North and Penelope's birthdays -- Kim Kardashian and Kanye West's daughter turned three on June 15, while Kourtney Kardashian and Scott Disick's little girl will turn four on July 8-- going all out on Saturday in the girls' favorite theme: mermaids! 
The cute cousins looked adorable in matching mermaid costumes at the party, which included pool toys, princesses and two cakes. Much of the family was decked out in mermaid gear as well, and, of course, they shared pics of the whole thing on social media. 
Kylie Jenner kicked things off, giving us a peek at the party's decorations via Snapchat.

The Kylie Lip Kit creator then documented North enjoying some princess time before getting in on the action herself.
Kylie, 18, even snapped a pic with the birthday girls and their beloved Disney princesses.
Khloe Kardashian had fun hanging out with Jenner and Scott Disick outside, showing off her pink wig on Snapchat. "This is how Scott and I spend our Saturdays," Khloe told her followers. "Becky with the pink hair." The 31-year-old reality star later revealed on her Instagram that her hair color was a request from North and Penelope.
Photo: Snapchat
"Normally it's Kylie in a wig!" Khloe joked in another snap.
Photo: Snapchat
Once the cake was ready, it was all about the Penelope and North.
Kanye and Kim (dressed in her 2014 mermaid Halloween costume) looked on as North prepared to blow out her candles.
Meanwhile, Penelope was all about posing for the camera.
The famous family ended the day with adorable snaps of Penelope, North, and Penelope's big brother, 6-year-old Mason, all posing with Aunt Khloe.

We just can't believe North and Penelope are another year older -- they're growing up so fast! Check out North's seven most adorable moments of the past year in the video below.