Viewers learn early on in the movie that Zach's diagnosis is terminal despite undergoing chemotherapy. Obviously, getting this news is completely gutting for Zach and his entire family. After being told he has a year to live, Zach decides to do everything in his power to spread joy to the people in his life.
Obviously, seeing a teenager — who should have his entire life ahead of him — grapple with his own death is hard to watch, and may cause children to reflect on their own mortality. It might be worth talking to your children about death, mentioning that there isn't always a tangible explanation for why sad things happen in life.
Although your children may cry at points throughout the movie, Clouds demonstrates how we shouldn't let a single day on earth go to waste.