Free Science Resources
There are a lot of free science resources online, and someday I will probably divide this list further and add so many more things. But let the below resources get you started.
Free Resources for Elementary Science
Later elementary (like, 3rd-5th grade, although kids both younger and older can often enjoy these resources) is the sweet spot for fun science. They have the patience (and, sometimes, dexterity) to do more complicated experiments, and they have the understanding to learn why things are the way they are.
PBS Kids has a great page of science videos for children.
Mystery Science is a subscription site, but they have a significant amount of free content. They have a rotating free science lesson on their home page, but if you search around, you'll find even more. Many of the projects have easy experiments that go with them. When I taught, I had a reputation as an amazing science teacher who was always doing interesting experiments, and my only secret to that success was Mystery Science.
The Boston Museum of Science has a great website with lots of experiments and information for families.
Free Resources for Middle and High School Science
My middle and high school, science tends to get a little more intense, but it can still be fun and hands-on.
The Royal Academy of Science has a really fun interactive website about the periodic table. If you're interested in elements, also check out Theodore Gray's photographs of the periodic tables.