1. The theme song says the word Caillou 14,356 times. And it's sung by him, in his voice, which will drive you nuts after five minutes. The director basically told the voice actor, "As high-pitched as you can go, with extra whine."
2. An early title sequence to the show used to feature Caillou throwing a temper tantrum. That happens every episode, and so of course your kids emulate that behavior.
3. That face. Here's what he pulls when he has to stay in bed because he's sick.
4. Caillou after he met baby Rosie for the first time.
6. His face after he ate a piece of olive bread baked by his grandmother. He spends the rest of the episode lying to her to make her think he likes olives. Even when he's trying to be considerate, he's an asshole.
7. Rosie exists for Caillou to be jealous of, to not share things with, and to occasionally play with.
8. Quick fact-check: He does not have cancer, so it's OK to bring the hate.
9. Back to business: The colour scheme is a nightmarish pastiche of every terrible children's illustration.
10. The adults dress like sexless blobs.
11. They don't even colour in the whole frame!
12. Even NFL player and parent Arian Foster had to tweet about how bad the show is.
13. And people were like "Yaasssss!"
14. The show's story structure is annoyingly complicated. That grating narrator? It's the voice of a grandmother reading Caillou stories to her two grandkids. So Caillou exists as a story within a story, just to be extra horrible to you.
It gets even more complicated later on, as explained by Crap My Kids Watch:
In later seasons, stories are introduced by his cat (in puppet form for some reason), who apparently can talk, but only in puppet form. In the story that the cat has introduced, the not-grandmother is still narrating, and the cat is now Caillou's not-special, not-talking, not-puppet cat. I don't get it at all.