SciShow: It is a great collection of science videos of different topics: historical science figures, science news, science talk show, science quizzes, most asked science questions.
SpanglerScienceTV: Steve Spangler teaches us tons of fun science facts and experiments - and all of them can be done at home!
asapScience: This channel uses fun illustrations to answer all of the questions that kids are dying to know like can you outrun a fart? and can you eat a bus?
BrainCraft: BrainCraft explains why we act the way we do, using psychology and neuroscience. You will find topics like How movies control your brain? How exercise can improve your memory?
Minute Physics: explaining science in a similar style as asapScience, but all topics are around physics. You will find topics like What is gravity? Is it better to walk or run in the rain? How an airplane is made? It thrives to explain science in the simplest way, and it does a good job in that aspect, so it is a great channel for kids of all ages.
Minute Earth: from the same creator of Minute Physics, but all topics are about our planet. Do you know Where did the water on Earth come from? Why is all sand the same?
Periodic Videos: all things chemistry. You will find a video about each element on the periodic table. There are new videos every week about science news, interesting molecules and other stuff from the world of chemistry.
Best of Science: A fun science channel covers a wide variety of science topics: physics, space, nature, chemistry, and more.
National Geographic: As their magazine, the YouTube channel covers many natural science videos.
Fizzics Education is a science outreach organization in Australia. They share their outreach efforts and science activities they do.
NASA: every thing is space related, from This Week at NASA, to Space to Ground, to ISS Expeditions.
Thanks iGameMom for these awesome suggestions!