WatchKnow is an organised collection of videos for children.

As the site explains

Imagine hundreds of thousands of great short videos, and other media, explaining every topic taught to school kids. Imagine them rated and sorted into a giant Directory, making them simple to find. WatchKnow–as in, “You watch, you know”–is a non-profit online community devoted to this goal.

This video explains exactly how the site works. So if you are looking for videos to support learning and teaching, WatchKnow is certainly worth checking out. Thanks to @joycevalenza for the link.