Kim is parent to North, 2-year-old son Saint and 10-month-old daughter Chicago with Kanye West, while her sister Kourtney Kardashian is parent to Penelope and two sons, 8-year-old Mason and 3-year-old Reign, with Scott Disick.
"Thankful for our babies being besties," Kim captioned a picture of North and Penelope posing cheek-to-cheek.
North and Penelope were born in June 2013 and July 2012, respectively.
The pair celebrate their birthdays at a joint party each year, and rang in the occasion with a unicorn-themed bash in June.
Kourtney previously told People North and Penelope being close in age helped foster their friendship.
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"Whenever [Penelope] sees [North] -- the second she walks into the room -- Penelope gets so excited," the star said.
"We never really had cousins that were close in age."
Kourtney shared a sweet moment with Penelope in a photo last week.
She and her children were forced to evacuate their home earlier this month due to the California wildfires.
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