"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," he wrote. "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."
He went on, "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."
"And I hope even in your dreams you can feel how much we love you," he added. "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."
Zuckerberg, 33, and Chan welcomed their first daughter Max in late 2015. She will be 2 in November.