My three sons weren't allowed to watch much TV, outside of PBS shows, when they were toddlers. When they discovered reruns of Theodore on the local PBS outlet they absolutely loved it. The stories were sweet and simple, teachy without being preachy, and they watched every episode. They also collected all the "action" figures (such as they were) and played with them for hours, and they even had a few easy-to-read books with stories about Theodore and his little tugboat friends.
By the time they turned four, my kids had outgrown the show, and stopped reading the books, just like they did with Thomas the Tank Engine and Jay-Jay the Jet Plane, and like we all did with Captain Kangaroo and Bozo, etc.
The host, a.k.a. the Harbour Master, was played with gentle grace by Denny Doherty of the Mamas and the Papas, who died today (1/19/07). He was wonderful on the show. May he rest in peace.