The answer may shock you. Click on this meme now for more information.

Caillou is bald because the Canadian creators of the show introduced him as a baby without any hair. When he grew older, the addition of hair made him unrecognizable, so they kept him bald. No, he does not have cancer.

This information was taken from the Official Caillou website: http://www.chouette-publishing.com/EN/why-caillou-bald