The writing is bad. The characters in the show say everything in the absolute simplest terms that it can be said, and then they will repeat it a couple of times and then the narrator will reiterate the point, just in case you missed it. Caillou, at times, seems completely unrealistic with the frequency and intensity of his bad behavior, ironically, often due to his own actions. (the famous episode in which Caillou thinks today is circus day.) This show is written as if it is a reality show or is somehow otherwise unscripted. In many episodes, the progression of the story is not linear or is completely broken, the two halves of the show not relating to each other. Lastly, much of Caillou's behavior goes unpunished, consistently leaving loose ends at the end of an episode.
The animation is worse. All of the characters are poorly drawn and the 'lines' with which they are drawn are 'Chunky', varying in width, like a bad pen that lightens, darkens and bleeds unexpectedly while drawing. The pallet is much too 'baby' oriented. These colors are what adults think of when a woman is decorating a nursery, not what the palette of an entire show. The backgrounds are almost never completely filled in, giving the impression that either the work was rushed, sloppy, done by a lazy man, or that Caillou's entire life is a dream. I have seen entire backgrounds that consisted of a blue block-like shape with rounded corners. The movement by most characters in most situations is extremely clunky and when objects in Caillou's world are manipulated, they quite often ignore the laws of physics.
The behavior is the absolute worst. Everyone in this show is a douche bag. Caillou is ALWAYS whining about something and often throwing tantrums, yelling, breaking things on occasion, and doing pretty much every unacceptable behavior that you can think of. His parents act like they are on Valium all the time. They often take no notice at all that Caillou is acting out in any way and NEVER punish him or take any action that might result in a lesson being learned. They are so aloof, in fact, that in one episode, the father leaves his 2 yr old on the changing table and exits the room completely, leaving the child unattended on top of the table. Rosie, Caillou's 2 yr old sister, seems to have a better grasp on the language than Caillou and throws less tantrums, however, she does throw objects and food at people, yell, scream, fight with her brother.
Basically, the average 'plan' for the episode is something happens, Caillou responds by acting out, his parents ignore his behavior and a couple minutes from the end, Caillou gets what he has been wanting the entire episode. The problem is that 4 yr-olds do not relate to Caillou the way the writers think they do. Rather, they imitate Caillou's behavior as he spends the majority of the episode trying for a goal and then acting out with every setback. The children don't follow the show close enough to really grasp the plot devices, so they see a kid whining and throwing a fit for 20 min and then getting what he wanted.
HORRIBLE SHOW all the way around.