Bill Nye is my monkeys primary science teacher. He is nice, has a great sense of humor (if you are under ten) and is knowledgeable about all science related subjects. He loves doing experiments and has an easy going natural manner when explaining why things work the way they do.
He has a huge series of videos most of which are available at your local library.
The Science Guy videos are great for five and up- especially if your monkeys are into science.
Stuff Happens are great videos for older kids and that the whole family can enjoy.