Los Angeles, California - When it comes to gifts for her four kids, Kim Kardashian prefers to keep things simple.

The 41-year-old reality star may be a billionaire, but she prefers to focus on sentimental value for gift-giving.

Kim revealed in a recent episode of On Purpose with Jay Shetty that she gives her children a letter to commemorate their birthdays each year.

"Every year, I write each one of my kids about a four or five-page letter on their birthday, about what the year was like, who their friends are, silly words they're saying, their favorite foods, all the silly things they do, and a little journey of what the year is like," she said.

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The SKIMs founder added that she hopes the letters will make her kids understand her and her parenting more, revealing that the letters she got from her own mom, Kris Jenner, did that for her.

Elsewhere in the interview, Kim didn't hold back when it came to discussing the many challenges of motherhood.

The middle Kardashian sister confessed that parenting is the most "challenging" thing she's ever done, even admitting that there have been nights where she cried herself to sleep.

Still, Kim clearly has a close bond with all four of her children: North (9), Saint (7), Chicago (5), and Psalm (3).

In the interview, The Kardashians star also touched on her marriage to their father, Kanye West, revealing that she doesn't view it as a "failure" but acknowledged that their conflicting values led to their split.

She shared that she hopes her children will focus on finding partners who align with their own values when they're older.