The BBC created Terrific Scientific and the videos cover lots of science topics and are aimed at Years 5 and 6
The National Geographic Kids YouTube channel has some great intro videos about all sorts of animals from all around the world. These would be ideal for KS1 classes, or possible lower KS2 if combining with habitat topics.
There is definitely a lack of really good online plant resources aimed at kids, however these Nature Boom Time videos are fantastic! We found that they made plants really exciting and definitely worth watching! Ideal for KS2 as they touch on more complex global issues as well as different types of plant, including some marine ones!
