Caillou is a Canadian educational children's television series which aired on Teletoon (both English and French versions) - with the first episode airing on the former channel on September 15, 1997 - until the fourth season. After that, it moved to Treehouse TV for season five. The series finale aired on October 3, 2010. It also airs on the PBS Kids Channel.
A Welsh dub, called Caio aired on Planed Plant/Planed Plant Bach and Cyw, the 3 preschool blocks of the Welsh channel S4C from 1999 to 2011.
Very little is known about the dub, and the only source mentioning the show ever airing on the channel is the Wikipedia page for Cyw. So far, no pages for the show have been found on Wayback Machine archives for Cyw/Planed Plant Bach's website.
in 2023, a docplayer thing about cyw was discovered that contained mention of the dub which confirms that it exists it can be found here: https://docplayer.net/21045258-Croeso-i-fyd-cyw-welcome-to-cyw-s-world.html