Not strictly childrens television, but i remember being off school for a couple of weeks with some childhood ailment, measles or mumps or something and to keep me quiet my mom used to put the tv on and at the time bbc were showing trade test transmission films for going over to colour tv, these were on for about 30 mins and were quite interesting, one i remember was called Cattle Carters which was transporting cattle across Australia, and another one was about a home made car a guy built from scratch, think it was called something like The Home Made Car.
There were lots of others as well, quite interesting and informative when you have read all your comics and drank all the Lucozade and nothing else to do.