Facebook CEO Mark Zuckerberg and his wife, Priscilla Chan, just had their second child, a daughter, named August. He has urged his young daughter to play outside and enjoy the wonders of being a kid, in a heartfelt letter posted on his social media site.

Zuckerberg posted a letter to his new daughter on Facebook. In it, he says "childhood is magical" and that August shouldn't be "worrying too much about the future." Zuckerberg says he and Chan are doing "everything we possibly can to make sure the world is a better place for you and all children in your generation."

Zuckerberg and Chan are not just paying lip service, either. Their Chan Zuckerberg Initiative has pledged to raise billions, including Zuckerberg's personal fortune from Facebook, to solve major global problems in the areas of health and education. One of the stated goals is to help cure, prevent or manage all human disease by the end of the century.

Dear August,
Welcome to the world! Your mom and I are so excited to see who you will become.

When your sister was born, we wrote a letter about the world we hoped she and now you will grow up in — a world with better education, fewer diseases, stronger communities, and greater equality. We wrote that with all the advances in science and technology, your generation should live dramatically better lives than ours, and we have a responsibility to do our part to make that happen. Even though headlines often focus on what's wrong, we still believe these positive trends will win out. We're optimists about your generation and the future.

But rather than write about growing up, we want to talk about childhood. The world can be a serious place. That's why it's important to make time to go outside and play.

You will be busy when you're older, so I hope you take time to smell all the flowers and put all the leaves you want in your bucket now. I hope you read your favorite Dr. Seuss books so many times you start inventing your own stories about the Vipper of Vipp. I hope you ride the carousel with Max until you've tamed every color horse. I hope you run as many laps around our living room and yard as you want. And then I hope you take a lot of naps. I hope you're a great sleeper. And I hope even in your dreams you can feel how much we love you.

Childhood is magical. You only get to be a child once, so don't spend it worrying too much about the future. You've got us for that, and we'll do everything we possibly can to make sure the world is a better place for you and all children in your generation.

August, we love you so much and we're so excited to go on this adventure with you. We wish you a life of joy, love and the same hope you give us.
Love,
Mom and Dad

Read more CNBC